Communication system with proactive network maintenance and methods for use therewith

ABSTRACT

A transmitter for use in a cable modem termination system includes a data processing module that generates a plurality of OFDM symbols from a data packet. A probe symbol generator generates a probe symbol, as one of a plurality of probe symbol types. The probe symbol is selectively inserted within the plurality of OFDM symbols, at a pre-defined probe symbol interval.

CROSS REFERENCE TO RELATED PATENTS/PATENT APPLICATIONS

The present U.S. Utility patent application claims priority pursuant to35 U.S.C. § 120 as a continuation of U.S. Utility application Ser. No.14/978,868, entitled “COMMUNICATION SYSTEM WITH PROACTIVE NETWORKMAINTENANCE AND METHODS FOR USE THEREWITH”, filed Dec. 22, 2015, whichis a continuation of U.S. Utility application Ser. No. 14/215,619,entitled “COMMUNICATION SYSTEM WITH PROACTIVE NETWORK MAINTENANCE ANDMETHODS FOR USE THEREWITH”, filed Mar. 17, 2014, issued as U.S. Pat. No.9,264,101 on Feb. 16, 2016, which claims priority pursuant to 35 U.S.C.§ 119(e) to U.S. Provisional Application No. 61/806,274, entitled“COMMUNICATION SYSTEM WITH PROBE SYMBOL TRANSMISSIONS”, filed Mar. 28,2013; U.S. Provisional Application No. 61/810,064, entitled“COMMUNICATION SYSTEM WITH PROBE SYMBOL TRANSMISSIONS FOR LEAKAGELOCATION”, filed Apr. 9, 2013; U.S. Provisional Application No.61/823,747, entitled “COMMUNICATION SYSTEM WITH PROBE SYMBOLTRANSMISSIONS OF DIFFERING TYPES”, filed May 15, 2013; U.S. ProvisionalApplication No. 61/859,370, entitled “COMMUNICATION SYSTEM WITH CARRIERWAVE SYMBOL TRANSMISSIONS FOR LEAKAGE DETECTION”, filed Jul. 29, 2013;U.S. Provisional Application No. 61/862,907, entitled “COMMUNICATIONSYSTEM WITH CARRIER WAVE TRANSMISSIONS FOR EGRESS MONITORING, PHASENOISE TESTING AND/OR MEASUREMENT OF SUBCARRIER SPACING”, filed Aug. 6,2013; U.S. Provisional Application No. 61/898,048, entitled“COMMUNICATION SYSTEM WITH PROACTIVE NETWORK MAINTENANCE”, filed Oct.31, 2013; and U.S. Provisional Application No. 61/949,098, entitled“COMMUNICATION SYSTEM WITH PROACTIVE NETWORK MAINTENANCE AND METHODS FORUSE THEREWITH”, filed Mar. 6, 2014; all of which are hereby incorporatedherein by reference in their entirety and made part of the present U.S.Utility patent application for all purposes.

BACKGROUND

Technical Field

The disclosure relates generally to communication systems; and, moreparticularly, it relates to point-to-multipoint communication systemssuch as cable modem systems.

Description of Related Art

In conventional point-to-multipoint communication systems, a networksupports bidirectional data communication between a central entity andmultiple customer premises equipment (CPE). Example point-to-multipointcommunication systems include cable modem systems, fixed wirelesssystems, and satellite communication systems. In each system, thecommunication path from the central entity to the CPE is typicallyreferred to as the downstream, while the communication path from the CPEto the central entity is typically referred to as the upstream.

One type of point-to-multipoint system is a cable modem system, whichtypically includes a headend that is capable of communicating withmultiple CPEs, each of which provides cable modem functionality. In acable modem system, the CPE can be a cable modem, a set top box, or acable gateway, to provide some examples.

DOCSIS (Data Over Cable Service Interface Specification) refers to agroup of specifications published by CableLabs that define industrystandards for cable headend and cable modem equipment. In part, DOCSISsets forth requirements and objectives for various aspects of cablemodem systems including operations support systems, management, datainterfaces, as well as network layer, data link layer, and physicallayer transport for data over cable systems. One version of the DOCSISspecification is version 2.0, and includes the DOCSIS Radio FrequencyInterface (RFI) Specification SP-RFIv2.0-I03-021218 (hereinafter “DOCSISRFI Specification”), the entirety of which is incorporated by referenceherein.

DOCSIS 2.0 supports the ITU-T J.83 B (hereinafter “Annex B”) standardfor downstream physical (PHY) layer transmissions from the headend tocable modems. Advances in communication technology are requiringincreasingly more bandwidth, which can lead to deficiencies in channelcapacity, especially with respect to these downstream transmissions. Forexample, even cable plants operating at a frequency of 750 MHz are beingchallenged with capacity shortages, due to increased demand for video ondemand (VOD), high-definition television (HDTV), digital services, andexpanding analog channel lineups. Numerous schemes have been proposed tohelp alleviate the downstream bandwidth issues, including analogspectrum reclamation and advanced video coding techniques. A DOCSIS 3.0specification with channel bonding support has been in use for severalyears and a DOCSIS 3.1 proposal has been circulated.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates an embodiment 100 of a communication system.

FIG. 2 illustrates an embodiment 200 of OFDM (Orthogonal FrequencyDivision Multiplexing).

FIG. 3 illustrates an embodiment 300 of a communication system.

FIG. 4 illustrates an embodiment 400 of a transmitter and receiveroperative to perform transmission of a signal.

FIG. 5 illustrates an embodiment 500 of an OFDM symbol stream with probesymbol insertion.

FIG. 6 illustrates an embodiment 600 of a quiet probe symbol.

FIG. 7 illustrates an embodiment 700 of an active probe symbol.

FIG. 8 illustrates an embodiment 800 of an active probe symbol.

FIG. 9 illustrates an embodiment 900 of an active probe symbol.

FIG. 10 illustrates an embodiment 1000 of an OFDM symbol stream withprobe symbol insertion.

FIG. 11 illustrates an embodiment of a network analyzer 1100.

FIG. 12 illustrates an embodiment of a trigger message block 1200.

FIG. 13 illustrates an embodiment 1300 of a cable plant with leakagesource 1302.

FIG. 14 illustrates an embodiment 1400 of a cable plant with leakagesource 1402.

FIG. 15 illustrates an embodiment 1500 of a leakage receiver 1525 andcentral terminal 1535.

FIG. 16 illustrates an embodiment 1600 of the location of a leakagesource via a plurality of leakage detection data.

FIG. 17 illustrates an embodiment 1700 of the location of a leakagesource in accordance with a cable plant map.

FIG. 18 illustrates an embodiment of a baseband processor or other dataprocessing element 440′.

FIG. 19 illustrates another embodiment of a leakage receiver 1404.

FIG. 20 illustrates an embodiment of a method.

FIG. 21 illustrates an embodiment of a method.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment 100 of a communication system. Inparticular, communication system 100 is a communication channel 199 thatcommunicatively couples a communication device 110 (including atransmitter 112 having an encoder 114 and including a receiver 116having a decoder 118) situated at one end of the communication channel199 to another communication device 120 (including a transmitter 126having an encoder 128 and including a receiver 122 having a decoder 124)at the other end of the communication channel 199. The respectivedevices 110 and 120 are operative to send and/or receive probe symboltransmissions for the purposes of determining the characteristics of thechannel 199, determining plant leakage and for performing otherfunctions including proactive network maintenance and optimization.

In some embodiments, either of the communication devices 110 and 120 mayonly include a transmitter or a receiver. There are several differenttypes of media by which the communication channel 199 may be implemented(e.g., a satellite communication channel 130 using satellite dishes 132and 134, a wireless communication channel 140 using towers 142 and 144and/or local antennas 152 and 154, a wired communication channel 150,and/or a fiber-optic communication channel 160 using electrical tooptical (E/O) interface 162 and optical to electrical (0/E) interface164)). In addition, more than one type of media may be implemented andinterfaced together thereby forming the communication channel 199.

It is noted that such communication devices 110 and/or 120 may bestationary or mobile without departing from the scope and spirit of thedisclosure. For example, either one or both of the communication devices110 and 120 may be implemented in a fixed location or may be a mobilecommunication device with capability to associate with and/orcommunicate with more than one network access point (e.g., differentrespective access points (APs) in the context of a mobile communicationsystem including one or more wireless local area networks (WLANs),different respective satellites in the context of a mobile communicationsystem including one or more satellite, or generally, differentrespective network access points in the context of a mobilecommunication system including one or more network access points bywhich communications may be effectuated with communication devices 110and/or 120. Any of the various types of coding described herein can beemployed within any such desired communication system (e.g., includingthose variations described with respect to FIG. 1), any informationstorage device (e.g., hard disk drives (HDDs), network informationstorage devices and/or servers, etc.) or any application in whichinformation encoding and/or decoding is desired.

FIG. 2 illustrates an embodiment 200 of OFDM (Orthogonal FrequencyDivision Multiplexing). In particular, an OFDM modulation scheme ispresented for use in conjunction with transmissions over communicationchannel 199 via devices 110 and 120. OFDM modulation may be viewed adividing up an available spectrum 202 into a plurality of narrowbandsub-carriers (e.g., lower data rate carriers). Typically, the frequencyresponses of these sub-carriers are overlapping and orthogonal. Eachsub-carrier may be modulated using any of a variety of modulation codingtechniques.

OFDM modulation operates by performing simultaneous transmission of alarger number of narrowband carriers (or multi-tones). Oftentimes aguard interval (GI) or guard space is also employed between the variousOFDM symbols to try to minimize the effects of ISI (Inter-SymbolInterference) that may be caused by the effects of multi-path within thecommunication system (which can be particularly of concern in wirelesscommunication systems). In addition, a CP (Cyclic Prefix) may also beemployed within the guard interval to allow switching time (when jumpingto a new band) and to help maintain orthogonality of the OFDM symbols.Generally speaking, OFDM system design is based on the expected delayspread within the communication system (e.g., the expected delay spreadof the communication channel).

FIG. 3 illustrates an embodiment 300 of a communication system. Aparticular embodiment of communication system 100 is presented as acable system 300 that provides bidirectional communication between aCMTS (cable modem termination system) 305 and a plurality of cablemodems 320 via a cable plant 310—specific examples of the devices 110,120 and channel 199 presented in conjunction with FIG. 1. In thisembodiment, the CMTS 305 and cable modems 320 operate in accordance witha DOCSIS protocol or other cable modem protocol that employs OFDMmodulation on the downstream link from the CMTS 305 and the cable modems320 and further the upstream link from the cable modems 320 to the CMTS305.

As discussed in conjunction with FIG. 3, the CMTS 305 and cable modems320 are operative to send and/or receive probe symbol transmissions thatinclude probe symbols 302 that are sent for the purposes of determiningthe characteristics of the cable plant 310, performing networkmaintenance and optimization, other purposes, etc. In particular, whilecurrent DOCSIS 2.0/3.0 provides for quiet time in upstream during whichthe noise floor is measured, in an embodiment, quiet probe symbols areinserted in the upstream and/or downstream transmissions to providesensitive measurements of quantities such as thermal noise, ingress, CPD(common path distortion), CSO (composite second order), CTB (compositetriple beat), products from laser and amplifier clipping, ringing ofprevious OFDM symbol into the quiet time, including echoes past thelength of the cyclic prefix, and optionally other measurements. In theaddition or in the alternative, active probe symbols can be inserted ineither the upstream or downstream transmissions to characterize thetransfer function of cable plant 310. In particular, the active probesymbols can be used to determine a complex frequency response (amplitudeand group delay), nonlinear response including amplifier compression,laser clipping, diode rectification effects, nonlinearities viahistogram techniques, as well as other characteristics of the cableplant 310 and/or characteristics of the individual transmitters andreceivers of CMTS 305 and each of the cable modems 320.

In an embodiment, actual data-carrying symbols may be used to fulfillthe function of active probe symbols 302. For this to be most effective,the contents of the data-carrying symbols are captured at thetransmitter so that they may be compared with the received samples tocharacterize the transfer function of cable plant 310. Such a datasymbol having known content, and used as a probe symbol 302, can bereferred to herein as a probe symbol. In all cases where a probe symbolis described herein, an actual data symbol can be used for this purpose.

In addition, the CMTS 305 and cable modems 320 are operative to sendand/or receive command data 304 and feedback data 306 related tocommands for analysis to be performed, and the results of the analysisincluding, for example, MIB (management information base) data and otherdata and commands.

This disclosure includes various embodiments for use with such as asystem 300. For example, a transmitter includes a data processing modulethat generates a plurality of OFDM symbols from a data packet. A probesymbol generator generates a probe symbol 302, as one of a plurality ofprobe symbol types. The probe symbol 302 is selectively inserted withinthe plurality of OFDM symbols, at a pre-defined probe symbol interval.The probe symbol may be a data symbol, in which case no special probeinsertion is needed, since a normal data-carrying symbol is used tofulfill the function of an active probe symbol. Rather, the contents ofthe data symbol are captured at the transmitter for later comparisonwith the received samples, which are also captured at the receiver.Synchronization is required to ensure that the same symbol is capturedat the transmitter and receiver. Such synchronization may be provided bya trigger message as described in conjunction with FIG. 12.

In another example, a network analyzer is configured to communicateproactive network maintenance data with a cable modem 320 and the CMTS305 to provide proactive network maintenance functions that include testand measurement of upstream and downstream parameters via probe symboltransmissions.

In a further example, a CMTS 305 or a cable modem 320 includes atransmitter that generates a plurality of OFDM symbols for transmissionvia the cable plant 310, wherein the plurality of OFDM symbols includesat least one pilot tone for cable plant leakage detection, phase noisetesting, the detection of sub-carrier spacing and/or other test andmeasurement purposes.

In yet another example, a transmitter for use in a CMTS 305 generates aplurality of OFDM symbols for transmission via the cable plant, whereinthe plurality of OFDM symbols includes at least one pilot tone forlocating leakage in a cable plant 310 associated with the CMTS 305,wherein the at least one pilot tone is a carrier wave pilot that isphase continuous over the plurality of OFDM symbols.

In yet another example, a transmitter for use in a CMTS 305 includes aprobe symbol generator that generates a probe symbol 302 for locatingleakage in a cable plant 310 associated with the CMTS 305. A multiplexerselectively multiplexes a plurality of OFDM symbols for transmission viathe cable plant 310.

In yet another example, a transmitter for use in a CMTS 305 includes adata processing module that generates a plurality of OFDM symbols from adata packet, wherein the data processing module pauses the generation ofthe OFDM symbols in response to a pause signal. A probe symbol generatorgenerates a probe symbol 302. A pause control generator generates apause signal. A multiplexer selectively multiplexes the probe symbol 302with the plurality of OFDM symbols, in response to the pause signal.

Further examples regarding the transmission, reception and analysis ofsuch probe symbols 302, command data 304 and feedback data 306 includingseveral optional functions and features are described in conjunctionwith FIGS. 4-21 that follow.

FIG. 4 illustrates an embodiment 400 of a transmitter and receiveroperative to perform transmission of a signal. In particular, atransmitter 480/receiver 490 pair are presented for use in conjunctionwith devices 110 and 120 in conjunction with communication channel 199,or more specifically a CMTS 305 and cable modem 320 communicating viacable plant 310 implementation of communication channel 199 or othercommunication system communicating via OFDM symbols. Input packets 420(that can include command data 304 and other data) are processed bybaseband processor or other data processing element such as basebandprocessor 440 to generate a plurality of OFDM symbols. As shown, thebaseband processor 440 includes functional modules that implement MACand convergence layer 402, FEC (forward error correction) encoding 404,IFFT (inverse Fast Fourier transform) 406, cyclic prefix insertion 408,and interleaver 410. The OFDM symbols 422 are selectively multiplexedwith probe symbol inputs 424 (such as probe symbols 302) from probesymbol generator 416. In a particular embodiment, a pause controlgenerator 418 generates a transmit enable/pause signal 425 that operatesto pause the generation of non-probe OFDM symbols 422 by basebandprocessor 440 and to selectively insert probe symbol inputs 424 into theOFDM output of multiplexer 412. This OFDM symbol stream is modulated andamplified via modulator 414 into an RF signal 426 for introduction ontothe cable plant 310. In addition or in the alternative, probe symbolinputs are otherwise input to one of more of the modules of a dataprocessing element such as baseband processor 440, such as MAC andconvergence layer 402, FEC (forward error correction) encoding 404, IFFT(inverse Fast Fourier transform) 406, cyclic prefix insertion 408, andinterleaver 410 or other module not specifically shown.

At the receiver 490, the RF signal 428 generated by RF signal 426 viatransmission through the communication channel 199, is amplified anddemodulated via demodulator 464 and demultiplexed 462 to separate theOFDM symbols 430 from the probe symbol outputs 432. The OFDM symbols 430are processed by baseband processor 442 into output packets 465 that caninclude recovered command data 204. As shown, the baseband processor orother data processing element such as baseband processor 442 includesfunctional modules that implement MAC and convergence layer 452, FEC(forward error correction) decoding 454, FFT (Fast Fourier transform)456, cyclic prefix removal 458, and de-interleaver 460. The OFDM symbols430 are selectively demultiplexed from the probe symbol outputs 432.

In a particular embodiment, a pause control generator 468, synchronizedwith pause control generator 418 based on a common timing scheme,generates a transmit enable/pause signal 435 that operates to pause thereception of OFDM symbols 430 by baseband processor 442 and toselectively route probe symbol outputs 432 into the PNM (proactivenetwork management) sample buffer 466 for further processing by ananalyzer 475 such as a PNM server or other processor that operates undercontrol of the command data 204 to analyze the probe symbol outputs togenerate feedback data 306 that, as previously discussed, characterizesthe cable plant 310 and/or provides other metrics. In addition or in thealternative, the analyzer 475 operates based on other data from thedemodulator 464, one or more modules from the data processor such asbaseband processor 442 and/or other portions of the receiver 490 andgenerates feedback data 306 based on the probe symbol transmissions orother metrics that, as previously discussed, characterize the cableplant 310 or other communication channel 199 and/or provides othercontrol and management information.

The feedback data 306 can be retransmitted via cable plant 310 or othercommunication channel 199 from a transmitter associated with receiver490 to a receiver associated with transmitter 480. In this fashion, aCMTS 305, can send probe symbols 302 and command data 304 to a pluralityof CMs 320 and receive feedback data 306 that is, in whole or in part,based on the probe symbol transmissions. In another mode of operation, aCM 320 can send probe symbols 302 and command data 304 to a CMTS 305 andreceive feedback data 306 that is, in whole or in part, based on theprobe symbol transmissions.

The optional use of the transmit and receive enable/pause signals 425and 435 on the data processing elements allow existing functional blocksto be implemented with minimal changes. In operation, the dataprocessing can be paused so the system will almost not know the pauseoccurred. For example, such as pause does not require flushing of theinterleaver since the interleaving process is paused as well. The onlyprocesses of the baseband processors 440 and 442 that care about thepause are those that depend on real time, such as time interpolationacross pilots, smoothing buffers, etc. The latency impact of such asystem can be minimal (for example, on the order of only 40-80microseconds. The transmit and receive enable/pause signals 425 and 435can be periodic signals to simplify synchronization. For example, thepause function can have a period of 1 second to 10 minutes, or be turnedoff if not needed. In particular, a shorter period, such as 1 second canbe employed for rapid data acquisition when in a troubleshooting mode ofoperation when troubleshooting a node. A longer time period such as 10minutes can be employed in a normal mode of operation where backgrounddata logging is enabled.

The baseband processors 440 and 442 and the analyzer 475 can each beimplemented via a single processing device or a plurality of processingdevices. Such a processing device may be a microprocessor,micro-controller, digital signal processor, microcomputer, centralprocessing unit, field programmable gate array, programmable logicdevice, state machine, logic circuitry, analog circuitry, digitalcircuitry, and/or any device that manipulates signals (analog and/ordigital) based on hard coding of the circuitry and/or operationalinstructions. The processing module, module, processing circuit, and/orprocessing unit may have an associated memory and/or an integratedmemory element, which may be a single memory device, a plurality ofmemory devices, and/or embedded circuitry of the processing module,module, processing circuit, and/or processing unit. Such a memory devicemay be a read-only memory (ROM), random access memory (RAM), volatilememory, non-volatile memory, static memory, dynamic memory, flashmemory, cache memory, and/or any device that stores digital information.Note that if the processing module, module, processing circuit, and/orprocessing unit includes more than one processing device, the processingdevices may be centrally located (e.g., directly coupled together via awired and/or wireless bus structure) or may be distributedly located(e.g., cloud computing via indirect coupling via a local area networkand/or a wide area network). Further note that if the processing module,module, processing circuit, and/or processing unit implements one ormore of its functions via a state machine, analog circuitry, digitalcircuitry, and/or logic circuitry, the memory and/or memory elementstoring the corresponding operational instructions may be embeddedwithin, or external to, the circuitry comprising the state machine,analog circuitry, digital circuitry, and/or logic circuitry. Stillfurther note that, the memory element may store, and the processingmodule, module, processing circuit, and/or processing unit executes,hard coded and/or operational instructions corresponding to at leastsome of the steps and/or functions illustrated in one or more of theFigures. Such a memory device or memory element can be included in anarticle of manufacture.

The pause control generators 418 and 468 can each be implemented via atimer, counter or other circuitry that generates a correspondingtransmit enable/pause signal 425 and receive enable/pause signal 435.

As discussed above, a channel estimation block analyzes the samples ofthe probe symbol outputs to, as previously discussed, characterize thatcable plant 310 or other communication channel 199 and/or provide othermetrics. Examples of channel characterization techniques and thegeneration of probe symbols 302 to support such techniques are providedbelow.

Example 1—Pilot Estimation

The technique operates by subtracting the values of scattered pilotsusing a smoothed channel estimate. The result is an estimate of thenoise floor at the pilot frequency, which moves across the whole band.The noise floor includes random noise and spurs. This can giveperformance 5-10 dB better than the required QAM SNR, e.g., over 40 dB.While this method is available in other receiver designs, as anenhancement, it can be used even in a broken channel, when only the PLC(Physical layer link channel) can be received by the CM 320. The CM 320can have to report the measurements via the upstream through the brokenplant condition as well. This can allow troubleshooting of brokenplants.

In this example, the noise estimation works as follows. The receivedsymbol on a scattered pilot bin is Y=H*X+N, where H is the channelresponse on scattered pilot (SP), X is the transmitted SP symbol, N isthe noise, all in frequency domain. If the receiver channel estimationH(ce) is sufficiently filtered, then N(est)=Y−H(ce)*X, N(est) is thenoise estimation on that pilot location. The noise power can be averagedover time to achieve better accuracy. N(est) includes receiverself-noise, spurs, implementation loss etc. The scattered pilotlocations can rotate through all bins. In this fashion, a noiseestimation can be generated for all bins in normal reception over time.Discrete CTB/CSO can also be easily detected if large noise power isseen at the known CTB/CSO frequencies.

Example 2—Silent Pilot Probe

At the narrowband extreme, sweep a single tone or a few tones, with zeromodulation, across the band. This is like having additional scatteredpilots with zero (silent or null) modulation. This approach can causeminimal disruption to an existing system design. This can provide betterperformance than using the existing pilots in Example 1 since thechannel estimate is not needed to subtract the pilot values, since thepilot values are zero. Considering a DOCSIS 3.1 implementation, the timeand frequency interleaver 410 can place nulled input sub-carriers onrandomized subcarrier locations across a number of frequencies andacross a number of OFDM symbols equal to the interleaver depth. So,rather than sweeping, the interleaver 410 provides nulled subcarriermeasurements at a randomized (but complete) set of subcarriers acrossmultiple transmitted symbols.

Example 3—Wideband (WB) Silent Probe

At the wideband extreme, pause downstream OFDM symbol stream every 1second to 10 minutes and insert a quiet symbol across the entire 192 MHzband. This can be the most sensitive measurement but can requiremodifications in the PHY and MAC design. An example of this approach ispresented in conjunction with FIG. 5.

Example 4—Noise Power Ratio (NPR) Probe

In this approach, less than the full band can be silenced. For example,a narrow band such as 6 MHz of contiguous tones can be silenced. Thisgenerates a notch that can be swept across the full band. The notch canfill with intermodulation products if nonlinearities were present in theplant. A challenge can be how to discriminate between the plant noisefloor and the intermodulation products. However, making the notch widercan help see the intermodulation products. An inverse nonlinearity inthe receiver can be adjusted until the notch is maximally open, possiblyusing the LMS algorithm to do the adjustment. Histogram techniques arevaluable for estimating nonlinearity of plant. This NPR method can aidwith the estimating. An example of this approach is presented inconjunction with FIG. 9.

Example 5—Data-Carrying Probe Symbols

As discussed in conjunction with FIG. 3, actual data-carrying symbolsmay be used to fulfill the function of active probe symbols 302. Inparticular, any type of data symbol can be used for this purpose. Forthis to be most effective, the contents of the data-carrying symbols arecaptured at the transmitter so that they may be compared with thereceived samples to characterize the transfer function of cable plant310 or support other proactive network management functions as describedherein. When the probe symbol is a data symbol, no special probeinsertion is needed. Rather, the contents of the data symbol arecaptured at the transmitter for later comparison with the receivedsamples, which are also captured at the receiver. Synchronization isrequired to ensure that the same symbol is captured at the transmitterand receiver. Such synchronization may be provided by a trigger messageas described in conjunction with FIG. 12.

Example 6—Reverse Interleaver Approach

In this example a silent (wideband or narrowband) probe is inserted. Inparticular, null QAM values are inserted at the input to theinterleaver. These values are scattered in a specific “reverseinterleaved” pattern such that the interleaving function can re-groupthem into contiguous tones across a single OFDM symbol. In response tothe transmit enable/pause signal 425, the blocks feeding the interleavercan know that these QAM slots were not available, and not insert datavalues into them. This can avoid the issue of pausing MAC and TC blocks,as the opportunities can be scattered in the normal data flow, ratherthan grouped. The PHY can still see a missing symbol or portion of asymbol. A state machine associated with the processing module such asbaseband processor 440 can be used to do the reverse interleavingmapping and associated control.

In an embodiment, the interleaver 410 includes a convolutionalinterleaver for subcarriers in successive OFDM symbols, henceinterleaving is continuous with no block boundaries to “pause” betweeninterleaver blocks. However it is possible to “pre-interleave” using thefact that for an interleave depth of N the adjacent subcarriers in anOFDM symbol are successively delayed by one symbol up to N successivesubcarriers and then reset and repeated (modulo N) until the end of theOFDM symbol. This approach can pre-interleave the null subcarriersacross N successive OFDM symbols in the reverse interleaving pattern. Inthis fashion, all the null subcarriers are generated at the output ofthe time interleaver in the same OFDM symbol entering the channel (thatis a single symbol with all null subcarriers). A subsequent frequencyinterleaver can randomize the subcarrier order in that symbol but thatis a known pattern that is reordered at the receiver. This can work inthe upstream as well in the proposed skew of mini-slots to reduceinterleaving depth by using adjacent mini-slots instead of subcarriers.This is analogous to pre-distortion, pre-equalization, or pre-coding.Pre-interleaving provides an elegant solution to quiet channelmeasurement.

In addition to the examples above, Example 5 can also be used to send awide band probe in a single OFDM symbol in the channel with eitherdownstream convolutional interleaving or upstream mini-slot skewinterleaving. The OFDM symbol builder which takes the input bitstreamand maps the data into the appropriate QAM symbols for the IFFT caninsert the test QAM subcarriers (null value of 0+j0 or a probe sequencevalue like PRBS BPSK values, complex sequence values, etc.) using astate machine that is synchronized with the phase of the convolutionalinterleaver commutator. This is only slightly more complexity than asimple counter. When triggered (i.e. in response to the “pause”), thetest QAM subcarriers are inserted (null value of 0+j0 or a probesequence value like PRBS BPSK values, complex sequence values, etc.) inthe most delayed paths in the first symbol, insert the next QAM testsymbol into the second to most delayed paths on the second symbol, andso on up to the first undelayed paths on the Nth symbol for aninterleaver with depth N. This state machine can be triggered at anytime to make a channel measurement without any framing required, justsynchronization with the interleaver phase to start the above process.The resulting interleaver output of the pre-interleaved OFDM symbolsequence can be a single null or probe symbol across all subcarriers.The CP is prepended and the IFFT modulates the symbol and transmits itinto the channel. On the receive side, the receiver looks for the testOFDM symbol with a marker, time stamp, MAC message or whatever iseasiest to avoid complexity of upper layer protocols. An FFT can recoverthe test OFDM symbol with all QAM test or null subcarriers. Thisso-identified symbol is considered null data by the demodulator. ThisFFT can be a separate processor as used for the full band capture frontend. That is, perform desired processing including or removing the CP,then average, window, compute MER, etc.

In an embodiment, the symbol constellations include null symbols. Fornull carriers, the interleaver and other blocks can have an extra bitindicating null value. For example, this is like having 257 QAM, insteadof normal 256 QAM, where the 257th point is a Cartesian zero, at theorigin of the constellation diagram, 0+j0. A silent symbol is just likeany other symbol, except the QAM constellation points are numerically0+j0. This last constellation point (zero) can be modulated onto allcarriers. RF muting can be better if it can be done, that is, turn offthe RF completely. However that is difficult to do without providingextra time for the RF circuits to settle. Muting each subcarrierdigitally by modulating it with a zero symbol, is more practical in somecircumstances.

In an embodiment pilots are turned off during a quiet probe symbol.Receiver algorithms operate on the same circumstances as when a symbolis missing, such as during a burst of noise in the channel. Further theRF automatic gain control can be frozen (paused) during the quietsymbol, whose arrival will be known in advance and indicated, forexample, by the receive enable/pause control signal 435. Further, insome circumstances, the absence of energy in the OFDM symbol can bedetected, with some delay. In addition, a probe symbol can be generatedthat is not fully quiet, but only some tones are quiet. The total powerof the probe symbol can be selected to be the normal power of an OFDMsymbol.

In an embodiment, there can be gaps in the PLC narrowband acquisition.In particular, DOCSIS 3.1 is designed to work with gaps. A quiet symbolcan be placed inside the gap with no impact on the PLC.

While delay through the Epoc PHY can generally be constant in a DOCSISimplementation, an occasional missing (quiet) symbol can be compensatedsmoothing the flow using a FIFO, and synthesizing input and output ratesof the FIFO using a rational NCO. For example, if 1/1000 of the OFDMsymbols are silent, synthesize two clocks with the ratio 999/1000.Another approach is to not burden Epoc with the silent probe issue, justuse it on DOCSIS 3.1.

In a cable system embodiment, having the input and output of the cableplant will permit use of “system identification” techniques. Thisinvolves having a model of the plant, including nonlinearities andfilter effects. One such model is:

lowpass filter→amplifier with compression→lowpass filter.

The parameters of the model can be adjusted to minimize the errorbetween the model and the actual data. For this to work, samples aretaken from the input and output of the plant. Output samples in thereceiver, can be provided to the channel estimation block such as a PNMserver for processing. Samples at the input to the plant, i.e., thetransmitted samples, can be obtained by either (a) remodulating theFEC-corrected subcarriers at the receiver, or (b) by having the headendsave the samples of a normal OFDM symbol that it transmitted. Eitherway, the input and output samples of the channel can be obtained. Formethod (b), a spec on the CMTS 305 transmitter can capture the samplesof a designated OFDM symbol. For method (a), a spec on the receiver canprovide the information necessary for the remodulation processing.

As previously discussed, a probe symbol 302 can occupy all OFDM tones inthe symbol, or only a partial number of tones. In one example, only partof the probe symbol 302 can be quiet. The total power can be normal soas not to affect analog AGC. This can allow investigation of harmonics,in the Example of FIG. 8 where tones from 200-250 MHz are on. A furtherembodiment can be to also leave the pilots on during the probe.

In an embodiment, histograms can be employed at different points(upstream and/or downstream) to provide useful orthogonal information tospectrum capture, if there is a set of problems with failure scenariosthat are hard to distinguish with spectrum data only. In model-basedlinearization, the nonlinearity of an amplifier is modeledmathematically with some number of parameters P1, P2, P3, etc. Themathematical model may include memory if the nonlinearity is a functionof the voltage history, and not just the instantaneous voltage. Themodel is inverted with some number of parameters Q1, Q2, Q3 . . .related to the parameters P. One method is to invert a remotenonlinearity with a local inversion block and local observation. Forexample, say there are several nonlinear amplifiers in the plant, eachwith some power series description. The cascade will also have a powerseries description. If the incoming waveform is observed and associatedwith a priori information to reconstruct the transmitted waveform, theparameters P and Q can be estimated and the nonlinearity inverted. Thisis easier if the transmitted signal statistics are known. Monitoring canidentify a broken amp in one of many paths. There are also enhancedcable modems (CMs) embedded in the hardline plant called DMONs(Downstream Monitors). These can be employed to capture histograms aswell.

In an embodiment, a counter approach can be employed to optionally pausetransmit and receive functions. The operation of such a counter approachcan be described in conjunction with the following analogy. Assume youare watching a movie, pause the DVD player to get up for a break, andcome back and start it again. With respect to the counter on the DVDplayer display, everything is smooth; it continues from where it leftoff with no glitch, and of course the sequence of frames in the film isunaffected by the break time. However if the DVD player tried to usereal time to present its video frames after the break, it can have tosubtract out the break time from the counter and things can getcomplicated. By using the counter, which paused along with the content,everything is smooth and the DVD player counter barely even knows thebreak occurred. So the idea of the pause button in the downstream isthat the MAC will not have to change anything except use the virtualcounter instead of the real-time counter.

These techniques can be applied to the processing blocks 440 as follows.Assume there are 3840 subcarriers in the system, and that an FEC encoder404 happens to end on subcarrier 500. In the normal case with no quietprobe, the FEC encoder 404 can start on say subcarrier 1900 of OFDMsymbol n and end on subcarrier 500 of OFDM symbol n+1. So, it can occupySC 1900-3840 of symbol n, and SC 1-500 of symbol n+1. In the case with aquiet probe inserted, the FEC encoder 404 can occupy SC 1900-3840 ofsymbol n, and SC 1-500 of symbol n+2. Symbol n can be quiet. Fromanother viewpoint, the pause functionality implies conceptuallymaintaining two symbol counters: a real-time symbol counter and avirtual symbol counter. The virtual counter does not count the quietprobe symbols, so there can be no gaps in its count sequence. Thevirtual counter can be used for FEC, etc.

The further operation of the transmitter 480 and receiver 490 inconjunction with the generation and transmission of probe symbols 302,command data 304 and feedback data 306 can be described in conjunctionwith the following additional examples.

Probe symbols 302 operate to permit measurement of cable plant 310 orother communication channel 199 and in particular a cable plant responseincluding underlying noise and interference. Both linear and nonlinearresponse of the cable plant 310 can be measured. The analysis of such aresponse can provide a wideband, short-duration view of the cable plant310 or other communication channel 199. In operation, the transmitter480, such as in a CMTS 305, transmits a downstream probe symbol 302 at apredefined interval that is programmable in the range of 1 second to 10minutes. The probe symbol 302 includes the following modes:

-   -   (a) Standard-pattern frequency-domain probe. A number of        standard test patterns are employed, such as 16 or some other        number. These standard test patterns include pre-defined        patterns that are defined by frequency domain values        corresponding to sub carrier QAM modulation values. A CMTS 305        inserts these samples at the input of IFFT 406 or at the mux        412. The CMTS 305 further inserts a cyclic prefix. In        particular, the baseband processor 440 accepts a control from        the probe symbol generator 416 to insert or not insert pilots.        When a probe symbol is inserted in this fashion, the baseband        processor 440 does not perform interleaving or FEC on this probe        symbol.    -   (b) Arbitrary time-domain probe. The CMTS 305 accepts a sequence        of time domain samples. The CMTS 305 inserts these samples, via        multiplexer 412 at the modulator 414 input as a replacement for        one entire OFDM symbol. In an embodiment, the CMTS 305 does not        perform pilot insertion, CP insertion, interleaving or FEC        encoding on this probe symbol.    -   (c) Noise power ratio (NPR) probe. The CMTS 305 accept values        for notch beginning frequency and notch ending frequency. The        CMTS 305 transmits a known test pattern in all subcarriers        except the notch subcarriers. The CMTS 305 transmits zero values        (zero RF or substantially zero RF) in the notch subcarriers. The        CMTS 305 inserts a cyclic prefix. The CMTS 305 optionally does        not perform pilot insertion, interleaving or FEC encoding on        this probe symbol.    -   (d) Quiet probe. The CMTS 305 transmits all zero samples (zero        RF or substantially zero RF) during the period of one full OFDM        symbol. In an embodiment, some subcarriers are left active, with        total symbol power maintained at normal level, to avoid a fully        quiet symbol.

In accordance with these examples above, the receiver 490, such as areceiver of a CM 320, captures the received probe symbol 302 andperforms the following processing via analyzer 475 to generate feedbackdata 306.

-   -   (a) Time domain sample capture. The CM 320 captures the time        domain (I and Q) samples corresponding to the probe symbol. The        CM 320 also captures additional samples ⅛ of a symbol before and        ⅛ of a symbol after the probe symbol, for a total of 1.25 times        the duration of the probe. The CM 320 can send captured samples        as feedback data 306 in response to a request from the CMTS 305        via command data 304.    -   (b) Spectrum. The CM 320 computes the FFT power spectrum of the        probe symbol using the same FFT size used for data reception.        The CM 320 can apply windowing to the spectrum. For example, a        Hanning windowing can be employed which is equivalent to complex        convolution in frequency domain with sequence [−¼, ½, −¼]. Other        windowing can likewise be employed. The CM 320 can perform        true-power averaging of spectra of multiple probes using, for        example, a leaky integrator with programmable time constant in        the range of 1 to 128 averages. The CM 320 can accept command        data 304 from the CMTS 305 to restart spectrum averaging. The CM        320 can provide feedback data 306 in the form of max-hold        spectrum showing maximum power value of each bin since last        reset. The CM 320 can send feedback data 306 that indicates the        latest averaged spectrum and/or max-hold spectrum to the CMTS        305 upon request, via command data 304.

In an embodiment, the analyzer 475 or CM 320 processes each type ofprobe symbol 302 to generate the following examples of feedback data306.

-   -   (a) Standard-pattern frequency-domain probe. The CM 320 can make        measurements on the probe symbol. Examples include power and        RxMER (receiver modulation error ratio) of each subcarrier; and        total received power, etc. RxMER can be computed using the known        constellation points of the probe pattern, not using decisions.    -   (b) Arbitrary time-domain probe. The CM 320 can make        measurements on the probe symbol. Examples include total        received power, etc.    -   (c) Noise power ratio (NPR) probe. The CM 320 can make        measurements on the probe symbol. Examples include: Ratio of        average power outside notch to average power in notch (excluding        4 bins at each edge of notch and excluding outlier bins 10 dB or        more above average); power and RxMER of each subcarrier; total        received power, etc.

In a further example, the CMTS 305 and CM 320 can cooperate tosynchronize probes in multiple OFDM bands. In particular, such aconfiguration permits a view of harmonics or other effects in one band,while a stimulus is provided in another band. The CMTS 305 transmits anactive probe in one band such as 200 MHz, and simultaneously transmits aquiet probe in another band, such as at the 3^(rd) harmonic, 600 MHz.The CM 320 captures and processes the probe in the quiet band, andprovides spectra, raw samples, etc. allowing the harmonic to be viewedand analyzed. Some leeway is provided in the synchronization of the twoprobes. This error can be removed in post-processing of the samples. Inoperation, the CMTS 305 includes the capability of transmitting probesymbols simultaneously in two OFDM bands. The CMTS 305 can synchronizethe probes in the two bands to an accuracy of, for example, +/−10 OFDMFFT clock periods.

In a further example, the CMTS 305 and CM 320 can cooperate to sharefrequency band measurements. In particular, such a configuration permitsmeasurement of intermittent noise and interference and/or provides anarrowband, long-duration view of a portion of the channel. An exclusionband is employed as a programmable contiguous set of subcarriers withzero modulation (zero RF). The CM 320 receives a list of start and stopfrequency bins, via command data 304, defining up to 16 bands across thereceive spectrum. The bands may or may not overlap. The CM 320 providesmeasurements for each defined band as feedback data 206. Examplesinclude: time-averaged power; max-hold power; time-averaged spectrum;max-hold spectrum; time-averaged power taken during intervals whenenergy or no energy is present (when power in band is above/belowdefined threshold), etc.

In a further example, the CMTS 305 and CM 320 can cooperate to share awideband spectral display. In particular, such a configuration provideswideband spectrum analyzer function in the CM 320 that can be reportedvia feedback data 206. The CM 320, via analyzer 475, provides widebandspectrum analysis capability per existing DOCSIS spectrum analysis MIB.The CM 320 can, for example, provide a spectrum analysis bandwidth of192 MHz or greater. The CM 320 may optionally provide a spectrumanalysis bandwidth covering the full downstream spectrum of the cableplant 310 or other communication channel 199.

In a further example, a CM 320 may provide a CMTS 305 value of the CM'sequalizer coefficients via feedback data 306—for example, in response toa request from the CMTS 305 via command data 304.

In a further example, the CMTS 305 and CM 320 can cooperate to share FECstatistics, or otherwise to monitor link quality by keeping statisticson FEC error events. The CM 320 can measure FEC statistics that areshared via feedback data 306. Examples include total number ofcodewords, number of codewords passing parity check, number of codewordsfailing parity check, errored seconds since last query, and count oferrors in each 1-second interval since last query up to 10 minutes, etc.This feedback data 306 can provide details of categories of errors suchas LDPC, BCH, codeword length, normal vs. shortened codeword, etc.

In a further example, the CMTS 305 and CM 320 can cooperate to generatea receive channel estimate such as a channel estimate that is computedby the receiver as part of its normal operation, based on pilots. The CM320 generates the receive channel estimate and sends the receive channelestimate as feedback data 306 to the CMTS 305 upon request, via commanddata 304.

In a further example, the CMTS 305 and CM 320 can cooperate to shareper-subcarrier RxMER and power measurements made by the receiver. Thispermits viewing the frequency dependence of the overall performance ofthe channel. Intent is to use granularity already provided in system, beit per-subcarrier, per-minislot, etc. In operation, the CM 320 makesRxMER measurements per subcarrier, and/or Rx power measurements persubcarrier. The CM 320 can provide average measurements of RxMER and Rxpower over all nonzero subcarriers, over zero-RF subcarriers, overpilots, and over data subcarriers via feedback data 306. Averaging canbe performed on true power and can be done with a leaky integratorfilter with programmable time constant of 1 second to 1 minute, or noaveraging. The CM 320 can send these measurements to the CMTS 305 viafeedback data 306 upon request via command data 304.

In a further example, the CMTS 305 and CM 320 can cooperate to share QAMconstellation for viewing and analysis. In an embodiment, since amountof data can be large, a reduced size record can be stored and sent tothe CMTS 305 via feedback data 306. The CM 320 can capture the receivedconstellation when commanded by the CMTS 305, via command data 304.Various modes can be used including:

-   -   (a) Full constellation. Soft-decision data can be shared from,        for example, up to 10 OFDM symbols. Size can be, for example, up        to 8K*10=80K complex numbers, 12 bits each on I and Q,        total=about 2 Mbit.    -   (b) Quantized constellation. A soft decision data can be        quantized, for example, to ⅛ the distance between constellation        points, that is, binned into a 2-dimensional histogram. Each bin        can be recorded, for example, as a 1-bit value. The effect is        that duplicates are removed, since once a location is “hit” it        is given the value 1, and if hit again it still retains the        value 1. The maximum number of points can be, for example,        4096*8*8=256K bits. A constellation can be accumulated for a        programmable number of OFDM symbols.    -   (c) Compressed constellation. This mode can include, for        example, up to 4096 constellation points with mean and standard        deviation of each inner point, plus up to 100 soft decision        samples lying outside constellation boundary. Size can be, for        example, 4096*2*12+100*2*12=about 100K bits.    -   (d) Error constellation. This mode can capture the difference        between the soft decision and the correct or nearest        constellation point. This can, for example, be done for a single        OFDM symbol. Maximum size can be, for example, about        8192*2*12=about 200K bits. The error constellation is most        accurate when data is known, such as pilots or zero-RF        subcarriers, or for probe symbols with known data pattern, as no        decision errors occur with known data.    -   (e) Compressed error constellation. This mode can capture the        difference between the soft decision and the correct or nearest        constellation point and, for example, only report the mean and        standard deviation of the error, plus up to 100 error samples        lying outside decision. Total size=2*12+100*2*12=about 2.5K        bits.

In a further example, the CMTS 305 and CM 320 can cooperate to share ameasurement of impulse/burst noise (timestamp, duration, level,correlation with FEC errors). The CM 320 can detect burst/impulse noiseevents above a programmable threshold and can timestamp the event usingthe mini-slot counter, with resolution of, for example, 1 sample of theOFDM FFT clock. The CM 320 can measure the duration of the event withresolution of 1 sample of the OFDM FFT clock and further can measure thetrue average power of the samples during the duration of the event. TheCM 320 can also timestamp FEC blocks containing errors so that they maybe compared to the timestamps of burst/impulse noise events. Any or allof this data can be provided as feedback data 306.

In a further example, the CMTS 305 and CM 320 can cooperate to share ahistogram of wideband samples—e.g. to provide a view of nonlineareffects in the channel such as amplifier compression and laser clipping.For example, this allows detection of laser clipping that causes onetail of the histogram to be chopped off, and replaced with a spike. TheCM 320 can capture the histogram of the time domain samples at thewideband front end of the receiver and share histogram information viafeedback data 306. The histogram can have a resolution of at least 256bins. The histogram can be two-sided, that is, encompass values from themost negative to most positive values of the samples. The histogram canbe accumulated over a programmable period of, for example, 1 second to 1minute, or until reset based on command data 304.

FIG. 5 illustrates an embodiment 500 of an OFDM symbol stream. Inparticular, an OFDM symbol stream 510 is shown graphically in time andfrequency. In the embodiment shown, a frequency range of 192 MHz can beemployed, however, other ranges can be used in other embodiments. Asshown, enable pause control 506, such as receive enable/pause signal 425or transmit enable/pause signal 435, indicates times in the OFDM symbolstream 510 where probe symbols 502 and 504 are inserted. In theembodiment shown, the enable/pause control 506 is a periodic signal.

In operation, the baseband processor pauses OFDM counters for one OFDMsymbol in response to the enable/pause control 506 to insert each probesymbol. For quite probe symbols, the transmitter of the CMTS 305, CM 320or other device transmits silence (zero) during this time period. Thereceiver at the CMTS 305 or CM 320 captures a number of samples, such as4K+ or 8K+ samples during this symbol period including the cyclic prefixand surrounding samples. These samples are buffered via the bufferedsamples and sent to the PNM server or other processor to measure andcharacterize noise floor, etc. of the communication channel 199, such ascable plant 310. For active probe symbols, the transmitter of the CMTS305, CM 320 or other device transmits a probe signal. The receiver atthe CMTS 305 or CM 320 captures a number of samples, such as 4K+ or 8K+samples during this symbol period including the cyclic prefix andsurrounding samples. These samples are buffered via the buffered samplesand sent to the PNM server or other processor to measure andcharacterize transfer function of the communication channel 199, such ascable plant 310.

FIG. 6 illustrates an embodiment 600 of a quiet probe symbol. Aspreviously discussed a quiet symbol can be generated by transmitting noRF or substantially no RF. During a quiet probe symbol 610, the receiversamples noise/interference by capturing PNM samples during a PNM samplecapture window 612 from before the end of a previous OFDM data symbol606 to after the beginning of a next OFDM data symbol 608. Theprocessing of these PNM samples allows measurement of ringing into andbeyond CP interval 620, such as echoes in the CP 622 and echoes outsidethe CP 624. The processing of these PNM samples also allows measurementof pre-ringing 626 into end of quiet probe symbol 610 as well as themeasurement of other parameters such as noise floor.

FIG. 7 illustrates an embodiment 700 of an active probe symbol. Whilethe example presented in conjunction with FIG. 6 was a quite probesymbol, non-quiet or active probe symbols may also be employed. Inparticular, active probe symbols can be used to characterize transferfunction of cable plant including complex frequency response (amplitudeand group delay), nonlinear response including amplifier compression,laser clipping, diode rectification effects, and/or other effectsgenerated via histogram techniques or other methodologies. These activesymbols can use some or all subcarriers for each OFDM probe symbol time.

In operation, the transmitter 480 can insert any desired RF samplesduring probe symbol time. For example, a frequency domain probe can begenerated that is wideband that uses all subcarriers or, in thealternative, some subcarriers may be muted to view harmonics andintermodulation products from active subcarriers. In a further example,a time domain probe can be employed. In particular, some portions of theprobe signal in time may be muted to view ringing of channel. The cyclicprefix may or may not be included in a probe symbol. In particular, a CPcan be included in circumstances where the probe symbol is intended tobe demodulated by receiver as a data signal.

In the example shown, a wideband probe symbol is generated. While thespectrum of the OFDM probe signal is substantially flat beforetransmission on the cable plant 310 or other communication channel 199,the channel introduces micro reflections that modify the spectrum afterthe channel.

FIG. 8 illustrates an embodiment 800 of an active probe symbol. In theexample shown, a narrowband probe symbol is generated. As shown, thecable plant 310 or other communication channel 199 introduces a −40 dbdistortion term at the third harmonic of the transmitted signal.

FIG. 9 illustrates an embodiment 900 of an active probe symbol. In theexample shown, a wideband probe symbol is generated with a notch. Inoperation, the notch fills with intermodulation products, and/or otherharmonics generated by the channel. As shown, the cable plant 310 orother communication channel 199 introduces a −40 db distortion term atthe notch frequency. The notch frequency can be swept to determineresults at different frequencies. More generally, any combination ofquiet bands and active probe symbols can be employed. In particular,transmitters can be synchronized via a common time reference or othersynchronization control to schedule a quiet probe symbol in one or moreOFDM bands at the same time as an active probe symbol in one or moreother OFDM bands. The captured received samples in the quiet band, alongwith knowledge of the active probe symbols that caused them, can be usedto analyze the impairment causing harmonics.

FIG. 10 illustrates an embodiment of probe symbol insertion. Asdiscussed in conjunction with FIG. 4, a (wideband or narrowband) probecan be interleaved prior to insertion into an interleaved probe symbol1000. In particular, null QAM values can be inserted at the input to theinterleaver 410 of FIG. 4. These values are scattered in a specific“reverse interleaved” mapping 1005 such that the interleaving functionof the interleaver 410 can re-group them into contiguous probe symbol1002 with contiguous tones across a single OFDM symbol. In response tothe transmit enable/pause signal 425, the blocks feeding the interleaver410 can know that these QAM slots were not available, and not insertdata values into them. This can avoid the issue of pausing MAC and TCblocks, as the opportunities can be scattered in the normal data flow,rather than grouped. The PHY can still see a missing symbol or portionof a symbol. A state machine associated with the processing module suchas baseband processor 440 can be used to do the reverse interleavingmapping 1005 and the associated control.

FIG. 11 illustrates an embodiment of a network analyzer 1100. Inparticular, a network analyzer 1100 is presented for use with a system,such as the system 300 described in conjunction with FIG. 3 thatincludes at least one CMTS 305, cable plant 310 and a plurality of cablemodems 320. While the CMTS 305 and CMs 320 are shown separate from thecable plant 310, it can be noted that the entire system 300 can beconsidered a cable plant. In particular cable plant 310 can representportions of the cable plant that are separate from CMTS 305 and CMs 320.It can also be noted that, while described as a cable plant or a DOCSIS3.1 compatible cable system, the various embodiments can be employed inother cable systems that include a CMTS 305 and CM 320. Also, thetechniques described herein can likewise be applied to other wired orwireless systems, and in particular, to other network elements andsubscriber devices used in such systems.

In operation, the network analyzer 1100 treats the system 300 as adevice under test (DUT) to monitor, test, analyze the performance ofsystem 300, and to generate test results in the form of reports andother test data. In an embodiment, any of the active and passive probesymbol transmission types described in conjunction with FIGS. 1-10 canbe used in this regard, however other testing can also be employed.

The operation of network analyzer 1100 and system 300 can provideProactive Network Maintenance (PNM). In particular, a plurality ofproactive network maintenance functions such as spectrum analyzerfunctions 1102, vector signal analyzer functions 1104 and other testpoint functions 1106 of CMTS 305 and cable modems 320 can be leveragedto enable measurement and reporting of network conditions such thatundesired impacts such as plant equipment and cable faults andinterference from other systems and ingress can be detected andmeasured. With this information cable network operations personnel canmake modifications necessary to improve conditions and monitor networktrends to detect when network improvements are needed. In one example,the system 300 can operate in accordance with a DOCSIS 3.1 PHYspecification.

As shown, FIG. 11 provides examples of the components, test points, andmanagement capabilities of the Proactive Network Maintenance provided inconjunction with network analyzer 1100 to monitor, test, and analyze theperformance of system 300. The CMTS 305 and CM 320 contain test points,which include functions of a spectrum analyzer 1102, vector signalanalyzer (VSA) 1104, and other test points 1106 to be used inconjunction with the network analyzer 1100. The goal is to rapidly andaccurately characterize, maintain and troubleshoot the upstream anddownstream cable plant, in order to guarantee the highest throughput andreliability of service. The spectrum analyzer functions 1102 can includefull-spectrum, narrow spectrum, notch spectrum or other spectralanalysis that is either triggered or un-triggered. The VSA functions1104 can include determining pre-equalizer and equalizer coefficients,constellation displays, RxMER vs. subcarrier measurements for DS and/orUS. Other test point functions 1106 include FEC statistics, impulsenoise statistics and/or histograms. Further functions performed by CMTS305, CM 320 and network analyzer 1100 are presented in conjunction withthe examples that follow.

The following downstream PNM actions define CMTS 305 and CM 320functions for obtaining and buffering symbol samples, triggeringcollection of upstream spectrum conditions information, providingwideband spectrum analysis, employing excluded subcarriers as a spectralnotch, providing equalizer coefficient values, providing QAMconstellation points for display, obtaining and reporting receiver MERmeasurements, obtaining and reporting forward error correctionstatistics, and reporting signal histograms for the downstream channel.

As discussed in conjunction with FIG. 3, actual data-carrying symbolsmay be used to fulfill the function of active probe symbols 302. Inparticular, any type of data symbol can be used for this purpose. Forthis to be most effective, the contents of the data-carrying symbols arecaptured at the transmitter so that they may be compared with thereceived samples to characterize the transfer function of cable plant310 or support other proactive network management functions as describedherein. When the probe symbol is a data symbol, no special probeinsertion is needed. Rather, the contents of the data symbol arecaptured at the transmitter for later comparison with the receivedsamples, which are also captured at the receiver. Synchronization isrequired to ensure that the same symbol is captured at the transmitterand receiver. Such synchronization may be provided by a trigger messageas described in conjunction with FIG. 12.

Downstream Symbol Capture

The purpose of downstream symbol capture is to provide partialfunctionality of a network analyzer 1100 to analyze the response of thecable plant. At the CMTS 305, the frequency-domain modulation values ofone full OFDM symbol before the IFFT, are captured and made availablefor analysis. This includes the I and Q modulation values of allsubcarriers, including data subcarriers, pilots, PLC preamble symbolsand excluded subcarriers. This capture can result in a number of datapoints equal to the FFT length in use (e.g., 4096 or 8192), 16 bits canbe used as the width for each of I&Q, with LSBs padded with zeroes ifrequired.

At the CM 320, the received I and Q time-domain samples of one full OFDMsymbol before the FFT, not including the guard interval, at the 204.8MHz FFT sample rate, are captured and made available for analysis. Thiscapture can result in a number of data points equal to the FFT length inuse (4096 or 8192), 16 bits in width for each of I&Q, with LSBs paddedwith zeroes if required. The capture can include a bit indicating ifreceiver windowing effects are present in the data.

Capturing the input and output of the cable plant is equivalent to awideband sweep of the channel, which permits full characterization ofthe linear and nonlinear response of the downstream plant. The MACprovides signaling via the PLC Trigger Message to ensure that the samesymbol is captured at the CMTS 305 and CM 320. In an embodiment, theCMTS 305 can be capable of capturing the modulation values of one fulldownstream symbol for analysis. In an embodiment, the CM 320 can becapable of locating and capturing the time-domain samples of one fulldownstream symbol for analysis.

Downstream Wideband Spectrum Capture

In an embodiment, downstream wideband spectrum capture provides adownstream wideband spectrum analyzer function in the DOCSIS 3.1. CM 320similar to the capability provided in DOCSIS 3.0. In an embodiment, theCM 320 can provide a downstream wideband spectrum capture and analysiscapability. The CM 320 can also provide the capability to capture andanalyze the full downstream band of the cable plant.

Downstream Noise Power Ratio (NPR) Measurement

The purpose of downstream NPR measurement is to view the noise,interference and intermodulation products underlying a portion of theOFDM signal. The CMTS 305 defines an exclusion band of zero-valuedsubcarriers which forms a spectral notch in the downstream OFDM signal.The CM 320 provides its normal spectral capture measurements, which showthe notch depth. The maximum notch width can be selected for example asa value that can normally not exceed 10 MHz. A possible use case is toobserve LTE interference occurring within an OFDM band; another is toobserve intermodulation products resulting from signal-level alignmentissues.

In an embodiment, the CMTS 305 can be capable of accepting start andstop subcarrier indices defining an exclusion band (notch). The CMTS 305can also set the modulation value of all subcarriers in the notch tozero (no energy).

Downstream Equalizer Coefficients

The purpose of equalizer coefficients is to provide access to CM 320downstream adaptive equalizer coefficients, which describe the linearresponse of the cable plant. The OSSI spec can define summary metrics toavoid having to send all equalizer coefficients on every query. In anembodiment, the CM 320 can report its downstream adaptive equalizercoefficients (full set or summary) for any single OFDM block uponrequest.

Downstream Constellation Display

The downstream constellation display provides received QAM constellationpoints for display. Equalized soft decisions (I and Q) at the slicerinput are collected over time, with optional subsampling to reducecomplexity, and made available for analysis. Start and stop indicesdefine the range of subcarriers which are included in the measurement.In an embodiment, only data-bearing subcarriers with the specifiedprofile and QAM constellation are sampled; pilots and excludedsubcarriers within the range can be ignored. 8192 samples can beprovided for each query—though a greater or fewer number can be used;additional queries may be made to further fill in the plot. In anembodiment, the CM 320 can be capable of capturing and reportingreceived soft-decision samples, for a single selected profile, singleconstellation, and selectable range of subcarriers within a single OFDMBlock.

Downstream Receive Modulation Error Ratio (RxMER) Per Subcarrier

The downstream receive modulation error ratio (RxMER) per subcarrierprovides measurements of the receive modulation error ratio (RxMER) foreach subcarrier. The CM 320 measures the RxMER using pilots andzero-valued subcarriers, which are not subject to symbol errors as datasubcarriers can be. Since scattered pilots visit all data subcarriers,and zero-valued subcarriers are located in defined locations includingexclusion bands, the RxMER of all subcarriers in the active OFDM bandcan be measured over time. The scattered pilot pattern overlaps the PLCpreamble symbols, which are used for the measurement as if they werepilots.

In an embodiment, only those zero-valued subcarriers which are processedby the CM 320 receiver are measured. For the purposes of thismeasurement, RxMER is defined as the ratio of the average power of theequalized QAM constellation to the average error-vector power. Forpilots, the error vector is the difference between the equalizedreceived pilot value and the known correct pilot value. For zero-valuedsubcarriers, the error vector is the unequalized received value itself,since the correct value is zero and there is no reliable channelestimate for excluded subcarrier locations with which to performequalization. Using this definition, the noise measurement of azero-valued subcarrier is expressed in terms of an equivalent RxMERvalue using the average QAM constellation power as a reference.

In one example of operation, for an ideal AWGN channel, an OFDM blockcontaining a mix of QAM constellations, including some zero-valuedsubcarriers, with 35 dB CNR on the QAM subcarriers, can yield an RxMERmeasurement of nominally 35 dB for all subcarrier locations includingthe zero-valued subcarriers. In an embodiment, the CM 320 can be capableof providing measurements of the RxMER for all subcarrier locations fora single OFDM Block, using pilots, PLC preamble symbols, and/orzero-valued subcarriers for the measurement. The CM 320 may omitmeasurements on some zero-valued subcarriers.

Signal-to-Noise Ratio (SNR) Margin for Candidate Profile

The purpose of this feature is to provide an estimate of the SNR marginavailable on the downstream data channel with respect to a candidatemodulation profile. The following algorithm can be used to compute thisestimate. The CM 320 only performs this computation upon request. Thesame computation is done for the NCP channel.

Algorithm:

-   -   (1) The CM 320 measures the RxMER value for each data subcarrier        as specified in above.    -   (2) From these measurements it calculates the average RxMER per        data subcarrier, MER1.    -   (3) It accepts as an input the required average MER per        subcarrier for the candidate profile, MER2.    -   (4) The SNR margin is defined as MER1-MER2, where all quantities        are in dB.        As an example, if the CM 320 measures MER1=33 dB, and the        candidate profile requires MER2=30 dB, the CM 320 reports an SNR        margin of 3 dB. In addition, the CM 320 reports the number of        subcarriers whose RxMER is at least x dB below the threshold of        CER=1e-5 for a given QAM order, where x is a configurable        parameter with, for example, a default value=3.        Downstream FEC Statistics

The purpose of the FEC statistics is to monitor downstream link qualityvia FEC and related statistics. Statistics are taken on FEC codeworderror events, taking into account both the inner LDPC code and outer BCHcode, and are provided on each OFDM channel and for each profile beingreceived by the CM 320. The measurements can be timestamped, forexample, using bits 21-52 of the 64-bit extended timestamp, where bit 0is the LSB, which provides a 32-bit timestamp value with resolution of0.4 msec and range of 20 days. Timestamping can be performed withnominal accuracy of 100 msec or better. In an embodiment, codewordcounts and codeword error counts can include only full-length codewords,i.e., having LDPC codewords of size 16,200 bits. Similar statistics canbe taken on the NCP, also only using full-length codewords, and on thePLC. MAC packet statistics are not profile-based, but are computed onall packets addressed to the CM 320.

The CM 320 can be capable of providing the following downstreamperformance metrics:

-   -   Uncorrectable full-length codewords: Number of full-length        codewords that failed BCH decoding.    -   Correctable full-length codewords: Number of full-length        codewords that failed pre-decoding LDPC syndrome check and        passed BCH decoding.    -   Unreliable full-length NCP Codewords: Number of full-length NCP        codewords that failed LDPC post-decoding syndrome check.    -   Unreliable PLC Codewords: Number of PLC codewords that failed        LDPC post-decoding syndrome check.    -   NCP full-length CRC failures: Number of full-length NCP        codewords that failed CRC check.    -   MAC CRC failures: Number of packets that failed MAC CRC check.    -   Total number of full-length FEC codewords.    -   Total number of full-length NCP codewords.    -   Total number of PLC codewords.    -   Total number of MAC packets.    -   Start and stop time of analysis period.

The CM 320 can be capable of providing the following downstream FECsummaries on each OFDM channel for each profile being received by the CM320:

-   -   Full-length codeword error ratio vs. time (seconds): Ratio of        full-length number of uncorrectable codewords to total number of        full-length codewords in each one-second interval for a rolling        10-minute period (600 values).    -   Full-length codeword error ratio vs. time (minutes): Ratio of        number of uncorrectable full-length codewords to total number of        full-length codewords in each one-minute interval for a rolling        24-hour period (1440 values).    -   Ending time of rolling period.    -   Red/yellow/green summary link status (colors defined in [DOCSIS        OSSI v3.1]).

The CM 320 can provide two collection and reporting methods for eacherror-count metric:

-   -   Long-term statistics. The CM 320 always collects metrics in the        background for each profile. The codeword (or packet) and error        counters are automatically reset once per hour. The previous        value of each counter is saved when the counter is reset due to        timeout, so that in a steady-state condition a full hour's        reading is always available.    -   Short-term statistics. The CM 320 performs a one-shot        measurement with two configured parameters, N_(e) and N_(c). The        CM 320 reports the results when N_(e) errors have occurred or        N_(c) codewords have been processed, whichever comes first. This        measurement is particularly useful for downstream profile        performance measurement.        Downstream Histogram

The purpose of downstream histograms is to provide a measurement ofnonlinear effects in the channel such as amplifier compression and laserclipping. For example, laser clipping causes one tail of the histogramto be truncated and replaced with a spike. The CM 320 captures thehistogram of time domain samples at the wideband front end of thereceiver. The histogram can be two-sided; that is, it encompasses valuesfrom far-negative to far-positive values of the samples.

In an embodiment, the CM 320 can be capable of capturing the histogramof time domain samples at the wideband front end of the receiver. Thehistogram can have a programmable accumulation period of 1 second to 1minute and a minimum resolution of, for example, 255 bins.

Downstream Profile Performance Metrics

The following measurements are used for both PNM diagnostics and forperformance testing of downstream modulation profiles. The data formatused to report a given measurement may differ for these two uses.

-   -   Uncorrectable full-length codewords    -   Corrected full-length codewords    -   MAC CRC codeword errors    -   NCP LDPC full-length codeword errors    -   NCP full-length CRC failures    -   Total number of full-length FEC codewords    -   Total number of full-length NCP codewords    -   RxMER per subcarrier    -   RxMER measurement type per subcarrier    -   SNR margin for candidate data profile    -   SNR margin for candidate NCP profile

The following upstream PNM actions set forth CMTS 305 and CM 320functions for obtaining and buffering symbol samples, providing widebandspectrum analysis, obtaining and reporting noise power measurements andstatistics, providing equalizer coefficient values, obtaining andreporting forward error correction statistics, and reporting signalhistograms for the upstream channel.

Upstream Capture for Quiet Period and Probe Symbol

The purpose of the capture is to view underlying noise and measure plantresponse, by capturing one or more OFDM symbols during a scheduled quietperiod or probe. A quiet period provides an opportunity to view theunderlying noise and ingress while no traffic is being transmitted inthe OFDM band. An upstream probe provides the partial functionality of anetwork analyzer, since the input is known and the output is captured.This permits full characterization of the linear and nonlinear responseof the upstream cable plant. A list of excluded subcarriers is alsoprovided, in order to fully define the transmitted waveform. The indexof the starting sample used by the receiver for its FFT is alsoreported. In an embodiment, the CMTS 305 can be capable of capturing thesamples of one upstream OFDM symbol, including guard time, during anupstream quiet period or probe, and making them available for analysis.

Upstream Triggered Spectrum Capture

The upstream triggered spectrum capture provides a wideband spectrumanalyzer function in the CMTS 305 which can be triggered to examinedesired upstream transmissions as well as underlying noise/interferenceduring a quiet period. The capture capability herein covers both OFDMand pre-DOCSIS-3.1 upstream channels that may exist in the UpstreamSpectrum.

In an embodiment, the CMTS 305 can provide wideband spectrum analysiscapability. The CMTS 305 can provide a spectrum analysis span coveringup to the full upstream spectrum of the cable plant. The CMTS 305 canprovide the ability to trigger the spectrum sample capture using thefollowing modes:

-   -   Free running    -   Trigger on SID (service identifier)    -   Trigger during quiet period    -   Trigger on mini-slot count.        Upstream Impulse Noise Statistics

Upstream Impulse noise statistics gather statistics of burst/impulsenoise occurring in a selected narrow band. In an embodiment, a bandpassfilter is positioned in an unoccupied upstream band. A threshold is set,energy exceeding the threshold triggers the measurement of an event, andenergy falling below the threshold ends the event. The threshold may beset to zero, in which case the average power in the band can bemeasured. The measurement is time-stamped using, for example, the D3.0field of the 64-bit extended timestamp (bits 9-40, where bit 0 is theLSB), which provides a resolution of 98 ns and a range of 7 minutes.

In an embodiment, the CMTS 305 can provide the capability to capture thefollowing statistics in a selected band up to 5.12 MHz wide:

-   -   Timestamp of event    -   Duration of event    -   Average power of event.        In an embodiment, CMTS 305 can provide a time history buffer of        up to 1024 events.        Upstream Equalizer coefficients

Upstream equalizer coefficients provide access to CM 320 upstreampre-equalizer coefficients, and CMTS 305 upstream adaptive equalizer(post-equalizer) coefficients, which taken together describe the linearresponse of the upstream cable plant for a given CM 320. The OSSI speccan define summary metrics to avoid having to send all equalizercoefficients on every query.

In an embodiment, the CM 320 can provide the capability to report itsupstream pre-equalizer coefficients (full set or summary) upon request.In an embodiment, the CMTS 305 can provide a capability for reportingits upstream adaptive equalizer coefficients associated with a given CM320 upon request.

Upstream FEC Statistics

Upstream FEC statistics provide for monitoring upstream link quality viaFEC and related statistics. Statistics can be taken on codeword errorevents. The measurement is time-stamped, for example, using bits 21-52of the extended timestamp. An LDPC codeword that fails post-decodingsyndrome check can be labeled “unreliable”, but the data portion of thecodeword may not contain bit errors; hence the “unreliable codeword”count can tend to be pessimistic. All codewords, whether full-length orshortened, are included in the measurements.

In an embodiment, the CMTS 305 can be capable of providing the followingFEC statistics for any single upstream user:

-   -   Pre-FEC Error-Free Codewords: Number of codewords that passed        pre-decoding syndrome check.    -   Unreliable Codewords: Number of codewords that failed        post-decoding syndrome check.    -   Corrected Codewords: Number of codewords that failed        pre-decoding syndrome check, but passed post-decoding syndrome        check.    -   MAC CRC failures: Number of packets that failed MAC CRC check.    -   Total number of FEC codewords.    -   Total number of MAC packets.    -   Start and stop time of analysis period.

In an embodiment, the CMTS 305 can be capable of providing the followingFEC summaries over a period of up to 10 minutes for any single upstreamuser:

-   -   Total number of seconds.    -   Number of errored seconds (seconds during which at least one        unreliable codeword occurred).    -   Count of codeword errors (unreliable codewords) in each 1-second        interval.    -   Start and stop time of summary period.    -   Red/yellow/green summary link status (colors defined in [DOCSIS        OSSI v3.1]).        Histogram

The purpose the histogram is to provide a measurement of nonlineareffects in the upstream channel such as amplifier compression and laserclipping. For example, laser clipping causes one tail of the histogramto be truncated and replaced with a spike. The CMTS 305 captures thehistogram of time domain samples at the wideband front end of thereceiver. The histogram is two-sided; that is, it encompasses valuesfrom far-negative to far-positive values of the samples.

In an embodiment, the CMTS 305 can be capable of capturing the histogramof time domain samples at the wideband front end of the receiver. Thehistogram can have a programmable accumulation period of 1 second to 1minute and a minimum resolution of 255 bins.

FIG. 12 illustrates an embodiment of a trigger message block 1200. Anexample format of a trigger message block (MB) is shown for use inconjunction with one or more functions and features described inconjunction with FIGS. 1-11.

In one mode of operation, the Trigger MB 1200 provides a mechanism forsynchronizing an event at the CMTS 305 and CM 320, such as the CM 320and CMTS 305 via command data 304 or other data exchange. In particular,the trigger message block 1200 can be used to trigger the implementationof a one or more of the PNM features previously described. In accordancewith this example, the CMTS 305 inserts a TR MB into the PLC andperforms an action at a specific time aligned with the PLC frame. Whenthe CM 320 detects the TR MB, it performs an action at the same relativespecified time aligned with the PLC frame received at the CM 320. Asdiscussed in conjunction with FIG. 3, when the probe symbol is a datasymbol, synchronization is required to ensure that the same symbol iscaptured at the transmitter and receiver. Such synchronization may beprovided by a trigger MB 1200, for example, via a Symbol Selectfunction.

The fields of trigger message block 1200 are shown in the table below inaccordance with an embodiment.

Field Size Value Description Message 4 bits 4 Trigger MB Block TypeTrigger 4 bits 1 Identifies type of action to Type perform Transaction 1byte Increments on each TR MB ID sent Trigger 2 bytes Group for unicast,multicast Group and broadcast triggers Frame 1 byte 2 How many frames towait Delay before performing action Symbol 1 byte 1 Which symbol in PLCframe to Select perform action uponIn this embodiment, the Trigger Type field identifies the type ofmeasurement to be performed. Value is unsigned integer from 0 to 15,with default=1. The Transaction Identifier field increments by one oneach trigger message that is sent, rolling over at value 255. Value isunsigned integer from 0 to 255. The Trigger Group field identifies whichgroup of CMs can respond to the trigger message.

In an example of operation, a CM 320 responds to the trigger message ifit has been configured as trigger-enabled and it has membership in thespecified Trigger Group. If the CM 320 has not been configured astrigger-enabled, it does not respond to trigger messages. The FrameDelay field tells the CM 320 how many frames to wait before performingthe specified action. Frame Delay=1 (not permitted) can indicate toperform the action in the next PLC frame after the frame containing theTR MB; Frame Delay=2 indicates to perform the action in the second PLCframe after the TR MB; etc. The value is an unsigned integer from 2 to31, with default=2. Values 0 and 1 are not permitted as they may notgive the CM 320 adequate time to prepare for the action.

The Symbol Select field tells the CM 320 which symbol in the specifiedPLC frame to perform the action upon. Symbol Select=0 indicates toperform the action on the OFDM symbol aligned with the first PLCpreamble symbol; Symbol Select=1 indicates to perform the action on theOFDM symbol aligned with the second PLC preamble symbol; Symbol Select=8indicates to perform the action on the OFDM symbol aligned with thefirst symbol after the PLC preamble, which corresponds to the first PLCdata symbol; and so on. The value is an unsigned integer from 0 to 127,with default=1. In addition to selecting a symbol, this parameter byconvention points to the time instant at the beginning of the selectedsymbol. (Note that, by contrast, DOCSIS PHY3.1 numbers the first PLCdata symbol as symbol 0, as it numbers the data field only, excludingthe preamble.)

When commanded to do so via a management object, the CMTS 305 can inserta single TR MB into the PLC. The CMTS 305 can position the trigger MB inthe PLC frame immediately after the timestamp MB but before any EM MBs,and before the MC MB. The CMTS 305 can increment the Transaction IDfield in each successive TR MB it sends. When trigger-enabled via amanagement object, the CM 320 can detect the TR MB.

For example, for a Downstream Symbol Capture measurement, the followingCMTS 305 requirements apply:

-   -   The CMTS 305 can set Trigger Type=1.    -   The CMTS 305 can capture and report the downstream symbol        specified in the TR MB.    -   The CMTS 305 can report the timestamp from the PLC frame pointed        to by the trigger message.    -   The CMTS 305 can report the Transaction ID.

For example, for a Downstream Symbol Capture measurement, the followingCM 320 requirements apply:

-   -   When it is trigger-enabled and a member of the Trigger Group        specified in the TR MB, the CM 320 can capture and report the        downstream symbol specified in the TR MB.    -   The CM 320 can report the Transaction ID.        Application of Trigger Message Block

This TR MB message 1200 can be used in accordance with the followingexample. In order for a CM 320 to respond to the TR MB, the CM 320 isfirst awakened if it is in sleep mode. The CM 320 is configured toenable triggering. The CM 320 is configured to belong to a TriggerGroup. The CMTS 305 inserts a single trigger message per measurementincluding a Trigger Group parameter associated with the group of CMsthat are intended to perform the measurement. The message is acted upononly by those CMs which are trigger-enabled and reside in theappropriate Trigger Group; unicast, multicast and broadcast groups aresupported.

In one mode of operation, the TR MB is to enable a Downstream SymbolCapture measurement. The goal of this measurement is to capture the sameOFDM symbol at the CMTS 305 and CM 320. The captured symbol is a normalsymbol (not a special test symbol or altered in any way) carryingdownstream QAM data traffic. The entire OFDM symbol is captured acrossall subcarriers, in the form of I and Q samples, at the CMTS 305 and CM320. The PLC frame is used only as a timing mechanism to define thelocation of the desired symbol in the downstream OFDM symbol stream. ForDownstream Symbol Capture, the Trigger Type parameter is set to 1.

An OSS management station, such as network analyzer 1100 or other OSSdevice, initiates the measurement via a write to a CMTS managementobject. The CMTS 305 inserts the TR MB in the PLC channel of thespecified OFDM downstream channel, waits the number of PLC framesdefined by the Frame Delay parameter, and captures the OFDM symbolspecified by the Symbol Select parameter. This capture can result in anumber of frequency-domain data points equal to the FFT length in use(e.g., 4096 or 8192), 16 bits in width for each of I&Q, with LSBs paddedwith zeroes if required.

A trigger-enabled CM 320 addressed by the Trigger Group parameterdetects the presence of the TR MB in the PLC, waits the number of PLCframes defined by the Frame Delay parameter, and captures the OFDMsymbol specified by the Symbol Select parameter. This capture willresult in a number of time-domain data points equal to the FFT length inuse (e.g., 4096 or 8192), 16 bits in width for each of I&Q, with LSBspadded with zeroes if required.

The CMTS 305 captures the 8-byte extended timestamp value present in thePLC frame in which the OFDM symbol was captured, and returns it to themanagement station along with the captured OFDM symbol samples; thisaids in identifying the captured data, and permits comparing the capturetime with other time-stamped events such as burst noise and FEC errors.The CMTS 305 and CM 320 both return the Transaction ID to the managementstation along with the captured data; this provides a mechanism forgrouping CMTS and CM data from the same symbol for analysis, and fordetecting missed captures. If no data was successfully captured by theCMTS 305 and/or a CM 320, that condition is reported to the managementstation in lieu of data, along with the Transaction ID if available. Thedata can stored locally in the CMTS 305 and CM 320, and returned to themanagement station based on a command issued by the management stationto a management object in the CMTS 305 and CM 320.

In an embodiment, an OSSI specification can limit how many Triggermessages can be sent before the captured data is read out from the CM320 by the OSS, in order to limit CM memory requirements. Therecommended initial default value is a maximum of one capture at a timein a given CM 320. If a new Trigger message arrives before the previouscaptured data has been read out, the CM 320 can optionally ignore thenew trigger and report that condition.

In an embodiment, the PNM system described herein synchronizes anupstream quiet time capture with a CMTS downstream symbol capture. Thereason is to measure common path distortion (CPD), that is,nonlinearities such as corroded connectors, which form diodes that causethe downstream signals to be modulated into the upstream.

In one mode of operation, these upstream and downstream measurements aresynchronized based on a timestamp value. For example, a PNM station suchas network analyzer 1100 or other OSS management station sends a commandto the CMTS 305 to trigger both captures. The MIB can ask for a captureduring a quiet time or upstream user transmission, and the CMTS 305 canconvert that into a timestamp value. In an embodiment, the DOCSIS 3.0timestamp (represented bits 9-40 of the DOCSIS 3.1 extended timestamp)can be used in this regard.

For example, the scheduler schedules a quiet period in the upstream inan arbitrary future mini-slot, and knowing the timestamp/mini-slotsnapshot offset, the timestamp value t1 corresponding to a symbol in themiddle of the quiet period is known. The CMTS 305 captures an OFDMsymbol period in both the upstream and the downstream when the PLCtimestamp=t1. In this fashion, a CMTS 305 can capture a downstreamsymbol at a predetermined time indicated by a known future PLC timestampvalue.

In a mode of operation, the CMTS software is operable to determine how atimestamp translates into a specific symbol on the PLC channel for thedownstream, and into a specific mini-slot count on the upstream. Inparticular, the CMTS software separately configures the downstream andupstream capture functions so that they are aligned. In this example,the CMTS and CM capture hardware in the downstream need not be aware ofthe timestamp value—these can still look for “symbol X after PLC frameY”. In this example, the CM 320 transmitter can set the trigger messagerelative to the timestamp to enable the synchronization.

In an example, the CMTS 305 provides synchronization of the US symbolcapture by providing a unique allocation of the quiet time for both theCM 320 and the CMTS 305. In particular, the CMTS 305 can use P-MAP(instead of the mini-slot) with a capture flag or a dedicated SID tospecify the symbol used.

In various embodiments, the burst receiver in the upstream channelcaptures more than one OFDM symbol (ideally 3-4 in succession includingguard times). This extra capture length can provide margin to accountfor any offset in the time alignment between upstream and downstreamsymbol boundaries and to ensure that the period of interest of theparticular OFDM symbol being synchronized is captured for analysis.

FIG. 13 illustrates an embodiment 1300 of a cable plant with leakagesource 1302. While the prior discussion has focused on the probe symboltransmissions for a broad range of functions including proactive networkmaintenance and network optimization, probe symbols and other OFDMsymbols can be inserted for detecting and/or locating a leakage source1302 in a communication channel 199 such as cable plant 310. The leakagesource 1302 can be an amplifier housing, connector, improper cablesplice or connection, break in a cable line, un-terminated cable, orother source of RF leakage from a cable, such as cable plant 310 orother channel 199. In particular, the leakage source 1302 is part of atransmission system where OFDM probe symbols are transmitted. Theleakage receiver 1304 operates by detecting these OFDM probe symboltransmissions.

In an embodiment, the probe symbols can be wideband probe symbols, suchas any of the active probe symbols presented in conjunction with FIGS.7-9. The leakage receiver 1304 includes a matched filter that is matchedto the active probe symbol or other signal processing to detect probesymbol transmissions. In one mode of operation, received signal strengthof the probe symbols is used by the leakage receiver 1304 to detect andlocate the leakage source. The leakage receiver 1304 optionally includesa directional antenna that is used to identify a direction from theleakage receiver 1304 to the leakage source 1302 to further aid inlocating the leakage source 1302.

Consider, for example, a probe symbol that occupies the full 192 MHzOFDM bandwidth for 1 OFDM symbol of length 20 microseconds. The form ofprobe symbol can be coherently received in a matched filter. It isequivalent to 3800 QAM values in a traditional OFDM transmission, eachwith 40 dB SNR inside the cable. Allowing 15 dB SNR to reliably detectthe signal presence in the leakage detector provides a processing gainof 40 dB-15 dB+10*log 10(3800)=61 dB to overcome the leakage path loss.

In a further mode of operation, the leakage receiver 1304 analyzes theleakage signals from leakage source 1302 as a function of frequency.This leverages the uncorrelated nature of LTE-band leakage and lowerfrequency (aeronautical band) leakage. Also the frequency content helpsindicate the mechanism causing the leak, as different size apertures(cracks, loose connectors, etc.) correspond to different wavelengths ofRF leakage energy. It can be noted that the leakage receiver 1304 can bea single purpose device or incorporated into an adjunct device that iscoupleable to a smartphone, table computer, laptop or other portabledevice or otherwise is incorporated in a smartphone, table computer,laptop, automotive receiver or other portable device.

FIG. 14 illustrates an embodiment 1400 of a cable plant with leakagesource 1402. Like the leakage source 1302, leakage source 1402 can be anamplifier housing, connector, improper cable splice or connection, breakin a cable line, un-terminated cable, or other source of RF leakage froma cable, such as cable plant 310 or other channel 199. In particular,the leakage source 1402 is part of a transmission system where OFDMprobe symbols are transmitted. The leakage receivers 1404, 1406, 1408and 1410 operate by detecting OFDM probe symbol transmissions.

In an embodiment, the probe symbols can be wideband probe symbols, suchas any of the active probe symbols presented in conjunction with FIGS.7-9. The leakage receivers 1404, 1406, 1408 and 1410 include a matchedfilter that is matched to the active probe symbol or other signalprocessing to detect probe symbol transmissions.

In addition to or as an alternate to operating as leakage receiver 1304,each of the leakage receivers 1404, 1406, 1408 and 1410 includes a GPSreceiver that provides both a stable time base and GPS location of thereceiver. When probe symbols are detected by the leakage receiver, atime of arrival (TOA) is calculated at each receiver and used inconjunction with the location of each receiver to pinpoint the locationof the leakage source 1402. Leakage data from each of the leakagereceivers 1404, 1406, 1408 and 1410 is collected by a central terminal1420, such as a master station, fixed station or other receiver and usedto calculate the location of the leakage source 1402. In particular, thecentral terminal employs similar techniques to GPS location—but with TOAdata from multiple receivers as opposed to TOA data at a receiver frommultiple sources.

FIG. 15 illustrates an embodiment 1500 of a leakage receiver 1525 andcentral terminal 1535. In particular the central terminal 1535 is anexample of central terminal 1420 and leakage receivers 1525 and 1525′present two examples of two leakage receivers 1404, 1406, 1408 and 1410.

The leakage receivers 1525 and 1525′ each include a leakage detectionreceiver 1504 having a matched filter 1508 that is matched to the activeprobe symbol to detect probe symbol transmissions in leakage signals1512. In addition, the leakage receivers 1525 and 1525′ further includea GPS receiver 1502 that provides both a stable time base for TOAcalculations and that processes GPS signals 1510 to generate GPSlocation data corresponding to the position of the receiver.

When probe symbols are detected by the leakage detection receiver 1504,a time of arrival (TOA) is calculated at each receiver via the TOAprocessor 1506. The TOA data and the corresponding GPS position areincorporated in leakage detection data 1520 and 1520′ that is sent viathe wireless transceivers 1508 to the central terminal 1535. As shown,the central terminal 1535 includes wireless transceiver 1528 forcoordinating the reception of, and receiving, the leakage detection data1520 and 1520′. While described in conjunction with wireless reception awired interface such as a universal serial bus interface, Ethernetinterface, an Internet connection or other interface, either wired orwireless can optionally be employed.

The central terminal 1535 further includes a processing unit thatexecutes leakage location application 1530 and a display device 1532that provides a graphical user interface and aids the user of centralterminal 1535 in identifying the location of a leakage source such asleakage source 1402. In operation, the leakage location application 1530operates based on leakage detection data from 2, 3, 4 or more leakagereceivers at multiple locations to pinpoint the location of the leakagesource 1402. Leakage data from each of the leakage receivers 1404, 1406,1408 and 1410 is used by the leakage location application 1530 tocalculate the location of the leakage source 1402. In particular, theleakage location application 1530 employs similar techniques to GPSlocation—but with TOA data and GPS coordinates from multiple receiversas opposed to TOA data at a receiver from multiple sources.

FIG. 16 illustrates an embodiment 1600 of the location of a leakagesource via a plurality of leakage detection data. In this example, X1,X2, X3 and X4 represent the coordinate positions of four leakagereceivers, such as 1404, 1406, 1408 and 1410. As discussed inconjunction with FIGS. 14 and 15, these coordinate positions can begenerated by a GPS receiver of each of the leakage receivers. The dashedcircle from each coordinate position (X1, X2, X3, X4) represents adistance from each coordinate derived from the TOA data generated bycorresponding leakage receiver and the corresponding speed of signaltransmissions in air. While the TOA data itself is non-directional, theleakage location application 1530 combines the leakage detection datafrom all four leakage receivers to calculate the location Y of theleakage source 1302. As shown, the location Y of leakage source 1302corresponds to the point of intersection of the four dashed circles. Aswill be understood by one skilled in the art, that while the presence oferrors in the GPS coordinate positions (X1, X2, X3, X4) and thecorresponding TOA data from each leakage receiver may dictate that anexact intersection may not be present, the identification of anintersection region and its midpoint may be used to estimate thelocation Y and further to provide a way to gauge the accuracy of theestimate.

FIG. 17 illustrates an embodiment 1700 of the location of a leakagesource in accordance with a cable plant map. In particular, the positionof certain components of a cable plant, such as cable plant 310, aresuperimposed on a layout map. In the example shown, dark lines representburied or overhead cable lines, and other symbols are used to representknown sources of possible RF signal leakage. In particular, squaresrepresent connectors and the triangle represents an amplifier housing.This cable plant layout map can be used in conjunction with the leakagelocation application 1530 to aid in identifying and locating a leakagesource 1402.

In one example of operation, the cable plant layout map is displayed ondisplay 1532 of central terminal 1535 to aid the user of the centralterminal in locating the leakage source 1402. In the example shown, thecalculated position Y is superimposed on the cable plant layout map. Theuser can visualize that the calculated location Y is near an amplifierhousing that can be the source of the leak. In a further example, theleakage location application 1530 automatically identifies likelysources of leakage in proximity to a calculated position Y andhighlights a likely source or several likely sources, as applicable, tothe user.

Returning again to FIG. 14, in another embodiment, egress monitoringsignals, such as any of the active probe symbols presented inconjunction with FIGS. 7-9 are inserted in downstream and/or upstreamtransmissions. The leakage receiver 1404 includes a matched filter thatis matched to the active probe symbol or other signal processing todetect probe symbol transmissions. In one mode of operation, receivedsignal strength of the probe symbols is used by the leakage receiver1404 to detect and locate the leakage source. The leakage receiver 1404optionally includes a directional antenna that is used to identify adirection from the leakage receiver 1404 to the leakage source 1402 tofurther aid in locating the leakage source 1402.

Consider, for example, a probe symbol that occupies the full 192 MHzOFDM bandwidth for 1 OFDM symbol of length 20 microseconds. The form ofprobe symbol can be coherently received in a matched filter. It isequivalent to 3800 QAM values in a traditional OFDM transmission, eachwith 40 dB SNR inside the cable. Allowing 15 dB SNR to reliably detectthe signal presence in the leakage detector provides a processing gainof 40 dB−15 dB+10*log 10(3800)=61 dB to overcome the leakage path loss.

In a further mode of operation, the leakage receiver 1404 analyzes theleakage signals from leakage source 1402 as a function of frequency.This leverages the uncorrelated nature of LTE-band leakage and lowerfrequency (aeronautical band) leakage. Also the frequency content helpsindicate the mechanism causing the leak, as different size apertures(cracks, loose connectors, etc.) correspond to different wavelengths ofRF leakage energy. It can be noted that the leakage receiver 1404 can bea single purpose device or incorporated into an adjunct device that iscoupleable to a smartphone, table computer, laptop or other portabledevice or otherwise is incorporated in a smartphone, table computer,laptop, automotive receiver or other portable device.

Consider a further embodiment where, in addition or as an alternative tothe active probe symbols, a plurality of phase-continuous OFDM pilottones are inserted in either the upstream or the downstreamtransmission. In particular, pilot tones such as continuous wave pilotscan be generated for egress monitoring, phase-noise measurement, thedetection of sub-carrier spacing and/or for other testing andmeasurement purposes.

In-band continuous pilot tones can be generated, which if properlychosen in frequency, result in true CW or substantially true CW, even inthe presence of a cyclic prefix. For example, the pilot symbols can betrue CW (unmodulated), that is, when viewed on a spectrum analyzer theycan be seen as spectral lines. The CW pilots can still be used as anOFDM pilot for acquisition and tracking, since the phase can be known.The CW pilot can remain orthogonal to the other OFDM tones. For egresstesting, link loss (leakage from cable, plus path loss) can be alimiting factor, not the transmit SNR of the CW tone. Hence in-bandpilot tones can optionally be implemented with no guard band. Thesein-band pilot tones can clearly be detected by leakage receiver 1404with time averaging over thousands of OFDM symbols.

In an embodiment, the CMTS 305 or CM 320 assigns a number of OFDM tones(5 to 10 for example) in either the upstream or downstream as carrierwave (CW) pilots to be continuously transmitted across all symbols. EachCW pilot can be the same as any other continuous pilots except for acontinuous-phase constraint across OFDM symbols. Since the cyclic prefix(and or guard interval) in each OFDM symbol adds a phase shift due toits run length, the CW pilot can take this additional phase shift intoaccount and start at the correct phase in the next OFDM symbol.

This form of pilot can be coherently received via a matched filter thatdetects the presence of these RF CW tones to measure plant leakage. Timeaveraging can be used in detection by the leakage receiver 1404 to bringthe tone up out of the OFDM background noise. In a further embodiment,leakage receiver 1404 can be synchronized to the OFDM frame, and the CWpilot tones can be detected based on their orthogonality to other OFDMsymbols. In this case, little or no time averaging may be necessary.

Consider the application of such CW pilots to a CMTS 305 and/or CM 320that operates in accordance with DOCSIS 3.1. Legacy sniffer equipment inDOCSIS 3.0 inserts tones which are about 9 dB lower in spectral densitythan the QAM signals in the cable plant, when measured in a 50 kHz BW,and which are located between QAM signals. In one mode of operation, thecontinuous pilots are boosted 6 dB above the OFDM data subcarriers. IfOFDM and legacy QAM spectral densities are about equal, the continuousphase CW pilots can be 9+6=15 dB stronger than legacy sniffer tones.This can be very effective for leakage detection—even in the presence ofLTE-band leakage and lower frequency (aeronautical band) leakage.

It can be noted that placing egress monitoring signals in the upstream(such as CW tones at known frequencies or an upstream full-band probe),as well as in downstream transmissions allow leaks to be quickly foundand repaired. Upstream transmissions can be traced to a given locationsince only one upstream Tx is transmitting a full-band probe at a time.This is better for locating the problem than downstream, where thesignal goes to all users.

In addition to the use of unmodulated pilot tones, some modulation canbe employed. For example, a constant value such as a point taken from aQAM constellation, AM, BPSK or other modulation technique can be used tomodulate the subcarrier. In one mode of operation, one large value of aconstellation can be used. In another mode of operation, the modulationcan be selectable between a set of programmable modulation values.

While the discussion above has focused on in-band pilots, thetransmitter can employ one or more tone generators to provideout-of-OFDM-band tones and/or OFDM-band-edge tones. For example, use 2,3, 4, . . . CW generators such as numerically controlled oscillators(NCOs) or other tone generators can be employed to generate pilot tones.If these pilot tones are generated on 50 kHz or 25 kHz centers orotherwise to match OFDM FFT bin spacing, then the pilot tones can beorthogonal with the OFDM symbols.

Consider a case where two or three pilot tones are generated. Byselecting the amplitude and phase of these tones, an AM signal can begenerated that can be easily received, demodulated and recognized byfield equipment such as leakage receiver 1304. With the CMTS 305 lockedto stable reference such as a DTI server and/or GPS, the AM signal canbe easily acquired with little frequency search. The use of such signalsfor egress monitoring promotes easy reception and measurement by simple,standard field equipment.

In addition to the use of such CP pilots discussed above for leakagedetection or other egress monitoring, the CW pilot tones can be employedin conjunction with CMTS 305 and CM 320 in the upstream and/ordownstream transmissions for phase noise testing, the detection ofsub-carrier spacing and/or other test and measurement purposes. Forphase noise testing, one CW generator may be sufficient to generate asingle tone for measurement on phase-noise test set or other equipment.For measuring OFDM subcarrier spacing, two tones can be generated withsubcarrier frequency spacing. The frequency difference can be measuredusing standard test equipment.

FIG. 18 illustrates an embodiment of a baseband processor or other dataprocessing element 440′. In particular, baseband processor or other dataprocessing element 440′ includes similar functions and featuresdescribed in conjunction with FIG. 4 that are referred to by commonreference numerals.

In particular, IFFT 406 and/or pilot insert block 1800 generates andinserts the OFDM pilot tones in the OFDM symbol stream 422′ fortransmission. For example, IFFT 406 can operate to insert in-band CWpilot tones. The IFFT 406 generates a number of OFDM tones (5 to 10 forexample) in the downstream as carrier wave (CW) pilots to becontinuously transmitted across all symbols. Each CW pilot is generatedthe same as other continuous pilots except for a continuous-phaseconstraint across OFDM symbols. Since the cyclic prefix in each OFDMsymbol adds a phase shift due to its run length, the IFFT 406 or pilotinsertion block 1800 can take this additional phase shift into accountand start the CW pilot at the correct phase in the each OFDM symbol forphase coherence across the sequence of OFDM symbols.

In one embodiment, the baseband processor or other data processingelement 440′ is optionally responsive to transmit enable/pause control425 to pause processing to insert probe symbols in the OFDM symbolstream. When such probe symbols are implemented, the CW pilots may ormay not be included.

Consider a mode of operation where tones are generated by sending a +1to the IFFT 406 for each of the subcarriers located at (in the complexbaseband) at 0 Hz and multiples of Nfft/CP within the band. For exampleif Nfft=4096 and CP=256, sending a +1 at subcarriers located at 0 Hz,16*50 KHz, 32*50 KHz, . . . results in discrete tones at RF withoutmodulation or time discontinuity. To double the number of possible validlocations for inserting a tone, a BPSK constellation can be employed andalternating values can be used (+1 and −1).

Consider a mode of operation for egress monitoring where CW pilots areinserted via IFFT 406 in the midst of the OFDM datasubcarriers—optionally with no guard band around the pilots. In thisapplication, time averaging can be applied in the leakage receiver 1304.Consider the example with a 4K IFFT, CP=256, and Window=128 and 15continuous pilots inserted with 2× pilot boosting; 8 on positivefrequency are set on subcarriers that generate CW tones, 7 on negativefrequency are set to subcarrier that ends up with a 180 degree inversionfrom symbol to symbol. The pilots are modulated by a BPSK sequence infrequency direction but static in the time direction. Simulation resultsfor a power spectral density of 1600 FFT symbol periods (note:1600*(4096+256) is approximately 7 million complex samples at 204.8 MHz)with 50 KHz RBW, indicate that the pilots show as +6 dB spikes abovenominal on positive frequencies and +3 dB above nominal on negativefrequencies. This indicates that CW pilots can be used with no guardband and will be clearly detectable by a receiver with time averagingover thousands of OFDM symbols. While described above without a guardband, a guard band that includes some number of subcarriers around eachCW pilot can likewise be implemented.

In another mode of operation, the IFFT 406 generates tones that aremodulated [+1, −1, +1, −1 . . . ] or [1, 1, 1, 1, . . . ] at 204.8 MHzat the IFFT output, which creates a tone at either Fc+102.4 MHz or Fcafter upconverting to a carrier frequency Fc. This scheme can bewell-suited for phase noise testing.

The optional pilot insert block 1800 can include a one or more NCOs forgenerating one or more out-of-OFDM-band tones and/or OFDM-band-edgetones that are added to the OFDM stream and summed with the output ofthe IFFT 406 after cyclic prefix insertion. This combined stream thatincludes the additional tones is modulated for transmission via amodulator, such as modulator 414 presented in conjunction with FIG. 4.While the pilot insert block 1800 is shown schematically as following CPinsert 408 and interleaver 410, other orderings are likewise possible.For example, the pilot insertion block 1800 includes 4 complex NCOsrunning on the 204.8 MHz clock summed to the output of the IFFT (afterCP insertion). This can be used for both testing phase noise and egressmonitoring. BSK modulation (+1, −1) can be applied to the tones or DC(no modulation).

Whether tones are generated in-band via IFFT 406 or by optional pilotinsert block 1800 as out-of-OFDM-band or OFDM-band-edge tones, thesetones can be used for cable plant leakage detection, phase noisetesting, the detection of sub-carrier spacing and/or other test andmeasurement purposes.

FIG. 19 illustrates an embodiment of a leakage receiver 1404. Theleakage receiver 1404 includes a plurality of matched filters 1908 thatare matched to the active probe symbols and/or CW pilot tones to detectprobe symbol transmissions in leakage signals 1912. Consider the examplediscussed in conjunction with FIG. 14 where the active probe symbolstake the form of phase-continuous at a number of OFDM pilots in thedownstream transmission. The matched filters detect the presence ofthese RF CW tones to measure plant leakage. Time averaging can be usedin detection by the leakage receiver 1404 to bring the tone up out ofthe OFDM background noise. In a further embodiment, leakage receiver1404 can be synchronized to the OFDM frame via optional framesynchronizer 1900 that operates in a similar fashion to correspondingportions of receiver 490 presented in conjunction with FIG. 4. In thiscase, the CW pilot tones can be detected based on their orthogonality toother OFDM symbols. In this case, little or no time averaging may benecessary.

Consider the case discussed in conjunction with FIG. 14 where two orthree pilot tones are generated. By selecting the amplitude and phase ofthese tones, an AM signal can be generated that can be received,demodulated and recognized by leakage receiver 1404 via AM detector1910. With the CMTS 305 locked to stable reference such as a DTI serverand/or GPS, the AM signal can be easily acquired with little frequencysearch. The use of such signals for egress monitoring promotes easyreception and measurement by simple, standard field equipment.

As discussed in conjunction with FIG. 18, CW pilots can be inserted viaIFFT 406 in the midst of the OFDM data subcarriers—optionally with noguard band around the pilots. Consider the case with a 4K IFFT, CP=256,and Window=128 and 15 continuous pilots inserted with 2× pilot boosting;8 on positive frequency are set on subcarriers that generate CW tones, 7on negative frequency are set to subcarrier that ends up with a 180degree inversion from symbol to symbol. The pilots are modulated by aBPSK sequence in frequency direction but static in the time direction.In this application, time averaging can be applied in the leakagereceiver 1404 in conjunction with matched filters 1908 that are tuned tothe frequencies of the inserted tones. In the alternative, traditionalFFT techniques can be used to detect the presence of the pilot tones inthe received spectrum.

FIG. 20 illustrates an embodiment of a method. In particular, a methodis presented for use with one or more functions and features describedin conjunction with FIGS. 1-19. Step 2000 includes generating aplurality of orthogonal frequency division multiplexed (OFDM) symbolsfrom a data packet. Step 2002 includes generating a probe symbol, as oneof a plurality of probe symbol types. Step 2004 includes selectivelyinserting the probe symbol within the plurality of OFDM symbols, at apre-defined probe symbol location to form a symbol stream fortransmission via a cable plant.

In an embodiment, the plurality of probe symbol types include one ormore types of an active probe symbol and/or a quiet probe symbol. Theplurality of probe symbol types can include a probe symbol for locatingleakage in the cable plant. The plurality of OFDM symbols include atleast one pilot tone for locating leakage in a cable plant associatedwith the CMTS. The at least one pilot tone can be a carrier wave pilotthat is phase continuous over the plurality of OFDM symbols. Theplurality of OFDM symbols can include at least one pilot tone for phasenoise testing and the detection of sub-carrier spacing.

FIG. 21 illustrates an embodiment of a method. In particular, a methodis presented for use with one or more functions and features describedin conjunction with FIGS. 1-20. Step 2100 includes generating a pausesignal. Step 2102 includes pausing the generation of the OFDM symbols inresponse to the pause signal, wherein the probe symbol is selectivelyinserted in the plurality of OFDM symbols, in response to the pausesignal.

It is noted that terminologies used herein such as bit stream, stream,signal sequence, etc. (or their equivalents) have been usedinterchangeably to describe digital information whose contentcorresponds to any of a number of desired types (e.g., data, video,speech, audio, etc. any of which may generally be referred to as‘data’).

It is noted that terminologies as may be used herein such as bit stream,stream, signal sequence, etc. (or their equivalents) have been usedinterchangeably to describe digital information whose contentcorresponds to any of a number of desired types (e.g., data, video,speech, audio, etc. any of which may generally be referred to as‘data’).

As may be used herein, the terms “substantially” and “approximately”provides an industry-accepted tolerance for its corresponding termand/or relativity between items. Such an industry-accepted toleranceranges from less than one percent to fifty percent and corresponds to,but is not limited to, component values, integrated circuit processvariations, temperature variations, rise and fall times, and/or thermalnoise. Such relativity between items ranges from a difference of a fewpercent to magnitude differences. As may also be used herein, theterm(s) “configured to”, “operably coupled to”, “coupled to”, and/or“coupling” includes direct coupling between items and/or indirectcoupling between items via an intervening item (e.g., an item includes,but is not limited to, a component, an element, a circuit, and/or amodule) where, for an example of indirect coupling, the intervening itemdoes not modify the information of a signal but may adjust its currentlevel, voltage level, and/or power level. As may further be used herein,inferred coupling (i.e., where one element is coupled to another elementby inference) includes direct and indirect coupling between two items inthe same manner as “coupled to”. As may even further be used herein, theterm “configured to”, “operable to”, “coupled to”, or “operably coupledto” indicates that an item includes one or more of power connections,input(s), output(s), etc., to perform, when activated, one or more itscorresponding functions and may further include inferred coupling to oneor more other items. As may still further be used herein, the term“associated with”, includes direct and/or indirect coupling of separateitems and/or one item being embedded within another item.

As may be used herein, the term “compares favorably”, indicates that acomparison between two or more items, signals, etc., provides a desiredrelationship. For example, when the desired relationship is that signal1 has a greater magnitude than signal 2, a favorable comparison may beachieved when the magnitude of signal 1 is greater than that of signal 2or when the magnitude of signal 2 is less than that of signal 1. As maybe used herein, the term “compares unfavorably”, indicates that acomparison between two or more items, signals, etc., fails to providethe desired relationship.

As may also be used herein, the terms “processing module”, “processingcircuit”, “processor”, and/or “processing unit” may be a singleprocessing device or a plurality of processing devices. Such aprocessing device may be a microprocessor, micro-controller, digitalsignal processor, microcomputer, central processing unit, fieldprogrammable gate array, programmable logic device, state machine, logiccircuitry, analog circuitry, digital circuitry, and/or any device thatmanipulates signals (analog and/or digital) based on hard coding of thecircuitry and/or operational instructions. The processing module,module, processing circuit, and/or processing unit may be, or furtherinclude, memory and/or an integrated memory element, which may be asingle memory device, a plurality of memory devices, and/or embeddedcircuitry of another processing module, module, processing circuit,and/or processing unit. Such a memory device may be a read-only memory,random access memory, volatile memory, non-volatile memory, staticmemory, dynamic memory, flash memory, cache memory, and/or any devicethat stores digital information. Note that if the processing module,module, processing circuit, and/or processing unit includes more thanone processing device, the processing devices may be centrally located(e.g., directly coupled together via a wired and/or wireless busstructure) or may be distributedly located (e.g., cloud computing viaindirect coupling via a local area network and/or a wide area network).Further note that if the processing module, module, processing circuit,and/or processing unit implements one or more of its functions via astate machine, analog circuitry, digital circuitry, and/or logiccircuitry, the memory and/or memory element storing the correspondingoperational instructions may be embedded within, or external to, thecircuitry comprising the state machine, analog circuitry, digitalcircuitry, and/or logic circuitry. Still further note that, the memoryelement may store, and the processing module, module, processingcircuit, and/or processing unit executes, hard coded and/or operationalinstructions corresponding to at least some of the steps and/orfunctions illustrated in one or more of the Figures. Such a memorydevice or memory element can be included in an article of manufacture.

One or more embodiments have been described above with the aid of methodsteps illustrating the performance of specified functions andrelationships thereof. The boundaries and sequence of these functionalbuilding blocks and method steps have been arbitrarily defined hereinfor convenience of description. Alternate boundaries and sequences canbe defined so long as the specified functions and relationships areappropriately performed. Any such alternate boundaries or sequences arethus within the scope and spirit of the claims. Further, the boundariesof these functional building blocks have been arbitrarily defined forconvenience of description. Alternate boundaries can be defined as longas the certain significant functions are appropriately performed.Similarly, flow diagram blocks may also have been arbitrarily definedherein to illustrate certain significant functionality.

To the extent used, the flow diagram block boundaries and sequence canhave been defined otherwise and still perform the certain significantfunctionality. Such alternate definitions of both functional buildingblocks and flow diagram blocks and sequences are thus within the scopeand spirit of the claims. One of average skill in the art will alsorecognize that the functional building blocks, and other illustrativeblocks, modules and components herein, can be implemented as illustratedor by discrete components, application specific integrated circuits,processors executing appropriate software and the like or anycombination thereof.

In addition, a flow diagram may include a “start” and/or “continue”indication. The “start” and “continue” indications reflect that thesteps presented can optionally be incorporated in or otherwise used inconjunction with other routines. In this context, “start” indicates thebeginning of the first step presented and may be preceded by otheractivities not specifically shown. Further, the “continue” indicationreflects that the steps presented may be performed multiple times and/ormay be succeeded by other activities not specifically shown. Further,while a flow diagram indicates a particular ordering of steps, otherorderings are likewise possible provided that the principles ofcausality are maintained.

The one or more embodiments are used herein to illustrate one or moreaspects, one or more features, one or more concepts, and/or one or moreexamples. A physical embodiment of an apparatus, an article ofmanufacture, a machine, and/or of a process may include one or more ofthe aspects, features, concepts, examples, etc. described with referenceto one or more of the embodiments discussed herein. Further, from figureto figure, the embodiments may incorporate the same or similarly namedfunctions, steps, modules, etc. that may use the same or differentreference numbers and, as such, the functions, steps, modules, etc. maybe the same or similar functions, steps, modules, etc. or differentones.

In a figure of any of the figures presented herein may be analog ordigital, continuous time or discrete time, and single-ended ordifferential. For instance, if a signal path is shown as a single-endedpath, it also represents a differential signal path. Similarly, if asignal path is shown as a differential path, it also represents asingle-ended signal path. While one or more particular architectures aredescribed herein, other architectures can likewise be implemented thatuse one or more data buses not expressly shown, direct connectivitybetween elements, and/or indirect coupling between other elements asrecognized by one of average skill in the art.

The term “module” is used in the description of one or more of theembodiments. A module implements one or more functions via a device suchas a processor or other processing device or other hardware that mayinclude or operate in association with a memory that stores operationalinstructions. A module may operate independently and/or in conjunctionwith software and/or firmware. As also used herein, a module may containone or more sub-modules, each of which may be one or more modules.

While particular combinations of various functions and features of theone or more embodiments have been expressly described herein, othercombinations of these features and functions are likewise possible. Thepresent disclosure is not limited by the particular examples disclosedherein and expressly incorporates these other combinations.

What is claimed is:
 1. A cable modem that communicates with a cablemodem termination system (CMTS) via a cable plant, the cable modemcomprising: a cable interface configured to receive, via a downstreamchannel from the CMTS, a plurality of orthogonal frequency divisionmultiplexed (OFDM) symbols of a data packet, and a physical layer linkchannel (PLC) message block that indicates a full OFDM symbol of theplurality of OFDM symbols; and a data processing module configured tocapture the full OFDM symbol received via the downstream channel ascapture data and wherein the capture of the full OFDM symbol receivedvia the downstream channel is synchronized via the PLC message blockreceived via the downstream channel.
 2. The cable modem of claim 1wherein the data processing module is further configured to generatefeedback data that includes the capture data and wherein the cableinterface sends the feedback data, via an upstream channel to the CMTS,for analysis.
 3. The cable modem of claim 2 wherein the analysisincludes characterization of linear and nonlinear response of thedownstream channel.
 4. The cable modem of claim 1 wherein a location ofthe full OFDM symbol in the data packet is determined via a symbolselect field of the PLC message block.
 5. The cable modem of claim 1wherein the capture data includes I and Q samples of the full OFDMsymbol.
 6. The cable modem of claim 1 wherein the capture data includesI and Q samples of the full OFDM symbol not including a guard interval.7. The cable modem of claim 1 wherein the capture data includes at leasta bit indicating if receiver windowing effects are present.
 8. A cablemodem that communicates with a cable modem termination system (CMTS) viaa cable plant, the cable modem comprising: a cable interface configuredto receive, via a downstream channel from the CMTS, a plurality oforthogonal frequency division multiplexed (OFDM) symbols of a datapacket, and a physical layer link channel (PLC) message block thatindicates a full OFDM symbol of the plurality of OFDM symbols; and adata processing module configured to capture the full OFDM symbolreceived via the downstream channel as capture data and wherein alocation of the full OFDM symbol in the data packet is determined via asymbol select field of the PLC message block received via the downstreamchannel.
 9. The cable modem of claim 8 wherein the data processingmodule is further configured to generate feedback data that includes thecapture data and wherein the cable interface sends the feedback data,via an upstream channel to the CMTS, for analysis.
 10. The cable modemof claim 9 wherein the analysis includes characterization of linear andnonlinear response of the downstream channel.
 11. The cable modem ofclaim 8 wherein the capture data includes time domain data.
 12. Thecable modem of claim 11 wherein the time domain data includes I and Qsamples of the full OFDM symbol.
 13. The cable modem of claim 11 whereinthe time domain data includes I and Q samples of the full OFDM symbolnot including a guard interval.
 14. The cable modem of claim 8 whereinthe capture data includes at least a bit indicating if receiverwindowing effects are present.
 15. A method for use in a cable modemthat communicates with a cable modem termination system (CMTS) via acable plant, the method comprising: receiving, via a downstream channelfrom the CMTS, a plurality of orthogonal frequency division multiplexed(OFDM) symbols of a data packet, and a physical layer link channel (PLC)message block that indicates a full OFDM symbol of the plurality of OFDMsymbols; and capturing the full OFDM symbol received via the downstreamchannel as capture data and wherein the capture of the full OFDM symbolreceived via the downstream channel is synchronized via the PLC messageblock received via the downstream channel.
 16. The method of claim 15further comprising: generating feedback data that includes the capturedata; and sending the feedback data to the CMTS for analysis; whereinthe analysis includes characterization of a response of the downstreamchannel.
 17. The method of claim 15 wherein the downstream channel issynchronized via the PLC message block based on a location of the fullOFDM symbol in the data packet determined via a symbol select field ofthe PLC message block.
 18. The method of claim 15 wherein the capturedata includes I and Q samples of the full OFDM symbol.
 19. The method ofclaim 15 wherein the capture data includes I and Q samples of the fullOFDM symbol not including a guard interval.
 20. The method of claim 15wherein the capture data includes at least a bit indicating if receiverwindowing effects are present.